I have discovered through many games, not only as necrons that having a solid theme in your army is essential to victory. Identifying that theme in an opponents army can often mean victory as well. By theme, i refer to the “style” of army. Obviously the two prevailing ones are shooting and assault, with a third being the hybrid. I would hope that descriptions of each is not required, merely that each chooses to engage and deal the majority of its damage a certain way. A list that's “all over the place” tends to do poorly, while one that's focused to one end does much better. Hybrid lists tend to feature strong elements from both.

With necrons, i have noticed that there can usually be a “sub-theme” - static or speed. A static list would entail things like large warrior blobs supported by ghost arks, whereas a speed list would bring in things like night scythes. Both bring the same strong shooting element into play. By deciding or identifying these 'themes' you can begin to build your force.

I like to start with the troop selections. Simply put, they are the backbone and workhorse of your army. You need to have 2 minimum, or even 4 with double force orgs. They are required to claim objectives in 100% of the scenarios, and the necron choices are both pretty strong. There are multiple builds for the necron troop section, but they generally involve one of 3 options:
MSU warriors in night scythes: - this can be quick strike and present a large force to the enemy, but small numbers means they die pretty fast, and large numbers of squads means they are unlikely to contain court upgrades. The benefit however, is claiming numerous objectives at once, and unlocking multiple night scythes.
Blob warfare: full size 20 man blobs of warriors or 10 strong of immortals, backed up by attached court lords and crypteks, and even ghost arks. Generally slow moving and quite pricey. The benefit here is obviously a rather resilient unit, that can take and dish a beating. However the downside is its mobility and pts cost. This route eliminates other support elements from joining your force.
Fast hammer: this one involves a moderate size selection, usually 5 immortals, or 10 warriors in a night scythe, with attached court lords. Very mobile, and very hard hitting, can take some punishment but not as much as the blob. Its a happy medium between the 2 above choices.

One other consideration in your troops is how to support them with court choices. This may seem academic, but the inclusion of one or more court members can make or break your squad. I wont delve into all of the nitty grittys here, but i will explore some of the basic options, whats good and whats perhaps best left elsewhere. For starters:
the lord. He can be kitted out pretty much like an overlord, and thus can bring some nice combat ability to a unit. His main role of orb caddy is almost mandatory. But by adding gear like mind shackle scarabs and tesseracts, he becomes a very dangerous challenge taker / maker and can make some foes think twice about charging.
Crypteks: these guys serve a nice support role in a shooting list, having access to some nice ranged gear and special war-gear. The veil of darkness and chronometron are leaders for special war-gear, some argue for the solar pulse... and other items. Personally i find limiting these special gear choices can keep your list in check, and prevent the points getting out of hand, as a heavy selection in the court adds up fast, and rarely pays off in the game. Some tricks that work however:
1 veiltek, + either 4 stormteks or a death mark squad. With the stormteks, you turn into a rather scary vehicle hunter. 16 haywire shots a turn, that are also str 5 makes them a threat to anything short of super tough MC. The death mark team is a very nice way of shifting off a tough squad hiding in cover (like an aegis line) due to combining the veilteks abyssal staff and the HfH rule. Adding a res-orb lord to any of these units just increases its survival and effectiveness, bringing a combat punch as well.
Finally, when it comes to troops, keeping the same 'theme' across your army can do wonders for you. Some would argue the merits of having multiple styles, such as a single blob camping “home base” along side a bunch of immortals in night scythes as mobile elements. The point I'm making here however, is that keeping your troops section generally doing the same thing brings a unity to your force and makes them stronger.

with the above information on how to select and take necron troops, and some basic pointers, here's some ways one can counter the the necron troop selection.

First: some things to know: ask whats in the unit. If you see a group of warriors, does it have a lord? Whats his gear? A cryptek? With what? Same for immortals. Ask if they are embarked in a night scythe or not. Even in tournaments, open lists are generally encouraged, so i would go as far to say as your allowed and entitled to know whats in the units your facing. Obviously there may be more immediate targets that need dealing with aside from the troops, but this will assume nothing else is presenting a need or threat.

So, for the warrior blob. These guys can put out a decent amount of Gauss fire, and if supported by a res-orb and ghost ark (or 3) can be hard to get rid of over a period of time, as they just keep standing up. The best way to deal with this, is simply not allow them to get up. Destroy the entire squad in one turn. Since the squad sports a 4+ armour save, and toughness 4, flamers and heavy flamers can do the trick quite nice. Also dropping a bunch of blast weapons on them can work as well. However you choose to do so, make sure the ENTIRE squad goes down, or at the very least flees. (as fleeing will remove any RP counters) obviously forcing a retreat is not a guarantee unless you have a way of imposing LD penalties This can sometimes be easier said then done, but with str 4 guns and limited range, they can be ignored for other more pressing targets, just keep in mind the Gauss fire CAN take out vehicles of any armour value with a 6 on the pen roll, and that's the main reason they show up in lists. A few things to counter them:
charge them with something that's durable, (good save) and can hit hard. A squad of termies, or something else with 2+/3+ armour, and a power-fist or power weapon, or a ton of attacks can tie up the unit in combat and while you'll be “tar pitted” for a few turns, you should win, and overtake them, or at the very least prevent them shooting up your armour.

Avoid them. They are generally slow, and not very mobile on foot, even with a ghost ark. With a night scythe they become more mobile, and with a veiltek they can teleport around. But if they show up on foot in a large blob, avoid their 24” range and pelt them with long range fire until ready to hit them en masse.

For immortals, the situation changes, but not by much. A better save, and a better gun, but lower numbers. Again, knowing what they bring matters. Gauss weapons will be used to target and shred vehicles, whereas tesla is more of an infantry target. (avoid charging tesla immortals... that over-watch can be nasty) the same concept applies here: when going after them, wipe them out to a man. Ap3 or better weapons, and massed fire can take care of them. If they bring a night scythe, shoot it down first chance. (i wont go into the debate about embarked passengers taking hits or not) but remember that the night scythe is only armour 11, no quantum shielding. But it does have living metal. A good round of anti flier fire should see one go down. Taking out the flier means they go back into reserve and generally walk on from their table edge.

Finally, something to consider in the troops section: is if they have lords, or crypteks attached. These can throw a kink into combating the unit, especially in assault. Be aware of these as many crypteks can have tricks to slow or harm those who would charge them. Lords present one of the more “annoying” threats – especially with mind shackles. Ill explore countering those later, but for now: avoid charging with a single large MC. MSS will just make it hit itself to death, followed by the war scythe finishing it off. Also avoid dropping any characters into the fight, and avoid challenges. (if your chaos, have a unit champ around to take the mandatory challenge) – and if you have higher initiative, when you charge don’t move into btb with his lord. Engage everything else, focus on the troops in the fight, and win the battle that way, force the lord to flee and overrun.

<end preamble>

and, with that critical info now explained, some more details.

First and foremost, the obvious:
doom scythes! These are the main heavy hitting favourite at the moment, probably because they are a HS slot flier, pack a tesla weapon, and a nice additional gun. The main strength is obvious, its great at adding some punch to a flier heavy list, and is respectable at both anti tank and anti infantry. For me, the biggest downside, is that its death ray is variable distance, (so not 100% reliable) and is generally used for taking out land raiders or other heavy armour vehicles. Right alongside the tesla destructor. While str 7, its not so great vrs av 12+ which is what your main weapon is shooting. And the main weapon really cant “mess up” a squad to the level the tesla destructor can. Its 2 great weapons, doing different things, but on the same vehicle. Some would argue that this just means versatility, i argue for inefficiency. I've used these in a few games, taking as many as 3 and each time i found one of the guns on it was not being used. And more often then not, i preferred the tesla destructor over the death ray, to which i think to myself: why not just bring more night scythes?

- in any case: some tips for using and bringing these guys down 1: keep together, with your other fliers, (you do have others?) but not too close. I find that flier maneuvering is tricky to do right, and people wonder why i don’t always fly off the table – and that's due to planning. This is obvisoulsy best shown in diagram, but ill try to dialogue it. I like formations of 3. have your lead one go about 24” forward from the corner of the deployment zone. Try to not point it more then 25-30 degrees towards the interior of the table, and zip up along the short edge. Do the same with your others, but stay back a few inches off the lead, and moving slightly towards centre with your starting position. You now have a nice formation that pretty much prevents enemy fliers getting in behind you. (ill explain that in a sec) turn 2, 45 degree turn, followed by 18” moves should bring you close to the other sides deployment end, still in arc, and still protected. Try to assume a triangle style formation. Essentially you can do this the whole game, just flying in circles around the table, next to its edges. As far as target priority, focus on the enemy aircraft first. Your death ray wont do jack, but the tesla will do wonders. If it looks like one is able to get behind you on its next move (consider your moves end point, vrs the other fliers current position and max move, and if it has hover or not) disengage. The best spot to be is behind the other guy, as its almost impossible for them to turn to face you and shoot back, barring hover mode. Obviously, as necrons don’t have hover, having an enemy flier lined up behind you shooting up your arse is a BAD thing.
(hey look! I just combined a how to use and how to kill all in one... sneaky sneaky..)

next up: the monolith. Its the big scary temple of “dear god shoot it now!” why? I have no idea. Maybe people see a gigantic pyramid show up, and think “death machine” or maybe its still got some of its rep from back in 3rd-5th. Either way, i believe its lost some of its charm, but is not useless. Here's some stuff to consider. First: its the only AV 14 in your army. This can be good and bad. Second, its the only real source of a nice big blast weapon with high str and low AP. Third, its a great area denial unit. People will generally try to avoid going near one, unless they have a ton of melta. So: I've found these guys work best when deep struck in, and when taken in 2 or more. 1 monolith using DS will get shot up and die. 2... well now that's less likely. 3.... and you have a whole lot of hurt, and not a lot of damage coming back. Not sure why this is, but it is. So, take 2 or 3 of these guys, and deep strike them in. (bring em in with zandrekhs phased reinforcements if you want) try to target an area about 14-20” away from the flank of the enemy, or its centre. You don’t want a mishap, and you definitely want to blast off that particle whip first opportunity. Drop them in, (hopefully all at once) and unload whips. Next turn, phase in your troops. Wraiths, immortals, lychgaurd, court, whatever. Now you have 6 units ready to beat face, directly up in your opponents comfort zone. You effectively give the other guy a choice: damage all 3, kill one, or ignore them. And they only get one turn of this, because next turn even more guys can teleport through, and continue the beating. And turn 2, what to shoot? The monoliths or the guys that just came out? - as far as countering the monolith, and the above trick, you’ll need as much lance, and melta as possible. Failing that, power fists in CC. Lazcannons CAN work...but its tricky and you need a lot. I would recommend focusing fire. And to prevent the DS trick, try to make it a risky deal. Deploy so that your force is together, and any DS close by WILL result in a large portion, if not ALL of your ranged and close anti tank beating it to a pulp. The monolith works best as a flank disruption unit, not a head on slugger. Force it into the head on role, and the necron player will be faced with either deploy conservatively, and go defensive (a win for the non cron) or be aggressive and risk it.

Next up: the super awesome annihilation barge. People love these things, and for good reason. 24” range on both its guns, both are tesla, and at str 6 and 7 its comparable str. Great for taking down masses of infantry, and light vehicles. And for only 90 pts. Plus, quantum shielding makes these guys pretty sturdy for the first few turns. To use them... well get within 24” and bake something. Not being fast, this presents a more slower support role, then a harassment unit but this also allows spiders and warriors to keep pace and provide a nice slow moving blob of OMGWHY incoming fire. I like to screen my barges with wraiths, or keep them close to my warriors, just to help spread out the incoming fire. Nothing screams “kill me now!” like a vehicle that just shot up a tac squad and its all by itself. If its grouped up, well now it might live longer. (plus having wraiths screen provides cover – and who cares if the other guy gets cover from your AP nothing shots) to bring these guys down or limit the effectiveness, its kind of a mantra, but focus fire. At AV 13 its as a little tricky to drop, but enough krak missiles will see it go down. Lazcannons also work nice. They are also slow, so faster moving things like fliers, fast skimmers, and JI can get in behind it (no QS there) and smoke it from behind. If all else fails, use haywire and glance the thing to death. - also remember its a 24” range, so if you can avoid it and make it chase... then that's almost just as good.

Tomb spiders: OK, so these guys to me are not really a heavy support slot. More of an elite choice taking a heavy spot on the force org. They don’t really pack any ranged power, and don’t have the hitting power in CC to really make them a HS slot. While they can contribute to the CC end of things as a MC,lack of an invun and only 3+ means they can be dropped pretty fast by most units. Don't get me wrong, they can contribute to the fight... but solo.. they tend to get knocked down pretty fast. However, their true strength lies in spitting out scarabs, and repairing vehicles.
the scarab thing: ill delve into this more with the fast attack section, but essentially a single spider can add additional models to a scarab unit, and while useful for keeping your scarab swarms up and going, and thus more effective, it requires the spider/s to be moving up WITH the scarabs, and getting into the middle of the fight. Not a great place for them. However, if your “style” is a rush up and eat things, and you have a very large amount of scarabs, then a large group of spiders is also a good addition. With 9 per forceorg chart, and at a pretty affordable base cost, you can really push up field with them. All those MC and scarabs that wont die (unless totally wiped) can overwhelm an armour based list that’s not prepared for them. To counter: spiders work best when supporting scarab swarms. So, take out the scarabs and leave the spiders to just 'derp' for a moment. The best way to deal with scarab swarms... nail them with a few blast weapons. Every army has some, be it frag missiles, battle cannons or anything. Obviously a str 6 blast will work the best, (as it doubles wounds, then instant kills. So 3 bases covered with such a hit would translate to 6 dead bases) if the scarabs are hiding in cover, smack them with a few flamers. These wont be as effective, but will still hurt, and negate any cover they have. Then, charge them with a unit of troops or other unit that wont care about the entropic strike, and tie them up. Scarabs want vehicles – they lack a bit vrs infantry. Be sure you WIPE THE UNIT in a single turn, else the nearby spiders will buff them back up to full power. While doing this, direct your heavy AT weapons on the spiders. Drop one, then move on. Anything with an ap of 3 or better is a good choice, and failing that str 6 or more. If you have high str attacks at good init, charge them. Otherwise stick to pelting them with things like plasma or melta.
The repair bugs. These are the more “optimal” method of using spiders in my books. Because the above scarab counter is so easy to achieve, and the only real way to avoid it is by hiding in cover, which is also a fairly easy counter, the second option is as a repair system. Taking 3 spiders with fabricator claws, you can instead support your other vehicles and keep them going. Repair hull points, fix immobilized or weapon destroyed results, and basically keep the vehicle working to top power. Combo this on a ghost ark group. The spiders hide behind the arks out of LOS and repair the arks, which in turn repair and replenish the front line warriors. To stop the “chain” the other guy has to get around you and kill the spiders. Hard to do if you deploy and hide the spiders properly. Plus, the spiders can counter charge and add a nice punch to a CC to help the warriors should they be charged by something. This can work with monoliths as well, ghost arks, any vehicle you expect to take a few hits, and want to keep working. The counter to this, is the same as the scarabs. Wipe the vehicle out in 1 turn. If its wrecked, it cant be fixed and the spiders just kind of 'derp' for a moment. Then you can focus some ranged fire on taking them down before they move on or otherwise make an annoyance of themselves.

The Doomsday Ark:
the other only pie plate in the army. And the longest range gun available. This one has some special requirements however, and i find that its best to build your list around these guys if you have them. While they can put out a very nice blast weapon at good range, they can only do this while static. So, to protect them and keep them alive, you’ll need a strong support element. This can be one of 2 things. Either a “castle up” to defend the ark from fast flankers or assaults, or you can go full forward press, and send everything to the enemy. Make them defend and while they do that, your arks blast them apart from a distance while they are otherwise occupied. Both have risk/reward, and i tend to prefer the later. Simply because necrons work best i believe when on the attack. For some reason, if they stay back and engage in a ranged fire fight, they tend to lack and get overwhelmed by the other army that can do the gun line better. But by pressing forward, you force the other side to play your game, and while still a firefight, your durability comes into play with a bit more of a “!!” on it, then it would otherwise. (rules wise its the same... but the actual effect does differ) for the arks themselves, you can pretty much snipe at whatever you want. Unfortunately, you will need to deploy them in a prime spot with good LOS and this opens you up to return fire – so try to kill those threats first to prevent your arks taking unwanted hits. Supporting the arks with tomb spiders to repair such hits increases survivability.
To counter them: take advantage of their static nature. Pelt them with long ranged anti tank fire, and send fast moving assault elements. Watch for spiders supporting them, as they can counter your charge and make things a bit tougher, but a good whack with a few power fists should drop them pretty fast. At 4 hull points, they can be a touch tougher then other vehicles, so pile on the hits any way possible, including lazcannons, missiles, lances, whatever. However, also remember that each ark only has 1 long range shot. Deploy in cover, and move out of its LOS, and you limit its power. Force it to move, and limit it more. This can allow you to defend against the forward press of the other necron units, or initiate an attack on the static defences and claim objectives.