Okay, here's a quick critique of what you have.
Dreadlord on Black Dragon:
This guy can tear things up in close combat... IF he doesn't get shot down first. Yes, the Dragon has a 3+ armor save. Cannons and stone throwers turn that into no armor save, and the dangerous shooting troops (handgunners and crossbowmen) turn that into a 50% chance. Using it requires planning, proper use of terrain... and plenty of other targets to dilute the attention of enemy shooting. Run him in a monster mash list with as many big gribblies as you can fit into the list. The two hydras you have aren't going to be enough.
Supreme Sorceress (I assume on foot):
This is your goto general. A level 4 with dispel scroll is almost mandatory for magic defense. Gaining all the lores (plus Dark magic) for casting is just icing on the cake. You'll want her in a medium-sized bunker unit, say 15-20ish. This unit shouldn't be too big; you want big units to get into combat... and that's the LAST place she wants to be, unless she's carrying the Black Amulet.
Many players forgo the dispel scroll for the Sacrificial Dagger, in a medium-sized unit of the cheapest possible troops (spears or swords). This allows you another chance to get a spell off if you fail to roll high enough on the power dice. Just be careful not to sacrifice so many troops you risk a panic check...
Hydras:
Last book these things were the BOMB. Now, not so much. They have 5 wounds, a 4+ armor save... and lost regeneration. The 'regrowing heads' thing doesn't even come close to replacing it since you only roll to regrow at the end of YOUR turn. Besides, as it takes wounds it becomes MUCH less of a threat. A hydra with 1 wound is only slightly more effective in combat than a Witch Elf. Again, these are just for dedicated monster mash lists.
Reaper Bolt Thrower:
A decent shooting unit... but it works better in teams. Still, it's a nice thing to have for shooting heavy cavalry and chaff-clearing.
Assassins:
Since they are now heroes instead of unit upgrades, they are now competing with Masters and Death Hags for points. The others are generally more broadly useful, where assassins tend to be very narrowly focused. Still, they can be used either to assassinate enemy characters (barring unkillable Chaos and Dwarf Lords) or clearing rank-and-file. They are generally used to babysit a vulnerable character or unit, giving much-needed combat capability to softer targets.
Cold One Knights:
The loss of the Hydra Banner AND the 'automatic' Fury of Khaine blessing in the new book REALLY hurt these guys. Casting is just too prone to being dispelled to be reliable, so the new Cauldron is no longer a good booster. So now the Cold One Knight - one of the most common recipients for that blessing - have lost between half and 2/3 of their old striking power. Which is a pity because these are really cool models.
Having said that, they still are useful. You can run 5 with musicians as part of your chaff/chaff hunters. They should destroy just about any chaff unit in the game on the charge, and with a 2+ save they won't take much damage in return. They also retain enough striking power to be a dangerous flanker to support a bigger unit hitting the enemy front. You can also run a largish unit (10+) as a fast combat block. They still hit hard, but they won't grind terribly well. Yes, they will have 3 S4 attacks and retain their 2+ armor save. They are still very limited on the number of wounds in the unit, and ultimately loose the grind to big blocks. So you need to pick targets you can either break on the charge, or get another ranked unit in to spread the attacks around.
All in all, they're more of a support unit than a main combatant. But S6 on the charge means they are your second most reliable way to deal with heavy armor (behind Executioners).
Repeater Crossbows:
These guys are really flexible. But don't let their main weapon confuse you into thinking their main function is shooting. Yes, they have 2
AP shots at
BS 4. That means they're hitting on 5's and 6's most of the game, so their accuracy is decidedly spotty. They are also only S3 shooting, so they struggle to inflict significant numbers of wounds on a large unit. (On the other hand, they're GREAT for shooting down underdefended monsters like Manticores, Griffins, Sphinxes, Heirotitans, Hydras, and the like-
AP reduces their saves to a less than meaningful level, and sheer number of shots means they'll do wounds fairly regularly. Give them a flaming banner and they massacre trolls.)
Yes, you can just take 10 with a musician as a chaff drop, and in that role they do a SUPERB job sweeping up enemy chaff - they are my goto 'broom' unit when I'm concerned about enemy chaff. But that neglects their primary role.
What repeater crossbowmen (with command and shields) really are is a backup close combat block every bit as potent as swordsmen... but they can start whittling wounds off an enemy unit 24" away instead of needing to get into close combat. So you run them wide for a turn or two for maximum shooting... then reform into a 5-wide combat block and march them up to provide ranks to an ongoing combat.
Spearmen:
Another unit that got hit in the new book. They nearly double in price to do the same thing... but now they're competing with Witches and 4+ armor Corsairs who do so much more for pretty much the same price. They are probably your best bet for a sorceress bunker, being fairly cheap and good enough fighters to deal with stuff that gets past the main battleline, but unless you go REALLY big (40+) they really don't belong on the main battleline themselves.
Corsairs:
A 4+ armor save makes them the most durable infantry we have. (Swordsmen and RXBs make an argument with their 5+ armor/6++ parry... but that parry does not apply to stomps, impact hits, shooting, flanking attacks, etc. and only outperforms the Corsair's 4+ on S6+ attacks... which aren't all THAT common.) But you need at least 10 for the unit, and 20-30 work better unless you're running an
MSU army.
Dark Riders:
With the ability to take shields and remain fast cavalry, these guys just became the default core choice. They are the best fast cavalry in the game, and are arguably the best chaff unit in the
DE list (harpies are cheaper, but do much less; Warlocks are over twice the cost, but are quite nasty for the price). With Vanguard, 18" march, and 24" shooting they can be picking wounds off enemy warmachines from turn 1 on. They can outmaneuver just about anything; they can flee then rally, move, and shoot; they can block up charge lanes until you want them open. Use these guys properly and they will win you games on a regular basis - not because of the damage they inflict directly, but by allowing YOU to dictate how the battle develops so you can match your strong units against the targets you want to fight while keeping
HIS strong units from ever earning their points back.
10 shots from 5 Dark Riders is also just about right to sweep up enemy chaff in a single volley. They can also chase fleeing stuff off the board with a potential 21" charge range. Nothing is funnier than seeing a fleeing
LD 10 killing machine lord chased off the board by a unit he could have wiped out with ease... had he not been running in the first place.
Black Guard:
With Witches now being core, the Black Guard has kind of lost their role. 2 S4 attacks is roughly equivalent to 3 S3 with poison except against really heavy armor... and then 1 S6 (Executioners or Cold One Knights) outperforms both. Yes, the Black Guard can take the Razor Standard and reduce 2+ to 4+. The Executioners can take it as well, and reduce that 2+ to 6+. Black Guard can shred light units with ease... but so can Witches and (to a lesser extent) Corsairs.
Really the only thing that makes the Black Guard valuable is being Stubborn, which means they'll pin an enemy unit down to their last man so long as the
BSB is nearby... but T3 and a 5+ save means that it likely won't take long for an enemy major combatant to carve through them. And as the MOST expensive infantry in the
DE book, that's a pretty hefty price to pay to pin an enemy unit down a turn (two at the most unless you run a REALLY big unit).
Executioners:
These guys are the dedicated can openers of our army. If it has better than a 4+ armor save, send in these guys. They'll carve it up and serve it for dinner as neatly as you please. Yes, they will take casualties. But swinging on I5 means they'll get to take their toll first (or at least simultaneously) except against
ASF or (some) characters. The combination of wounding on a 2+ and Murderous Prowess means nearly every hit WILL wound... and WS5 means they tend to hit a LOT.
But as I said, they WILL take casualties from EVERYTHING (this is true of all
DE infantry, but it's especially important against the expensive-yet-only-having-one-attack Executioners). Even skaven slaves will take a significant toll from Executioners. So you want to make sure these guys fight what YOU want them to fight, and not get tied up in chaff and tarpits. They WILL get swamped by sheer numbers eventually.
These guys work best in big blocks. Units of 5 are basically kamikazes looking to hit a cavalry unit in the flank and do damage all out of proportion to their numbers before they die... and Witches are better at that sort of thing in general. Not vs. heavy cavalry, granted, but 10 Witches will carve a rank or two off most infantry units. Kamikaze Executioners will likely only kill 3 models, so you want them to hit something nice and expensive.
Now I know I've kinda put down a lot of stuff in this critique. This is not to say that the units in question are bad, they are just not the best the
DE have to play with. Even Spearmen - the weakest unit in the book - can hold their own against equal points of... well, just about any other unit in the game. The
DE book can hold it's own against any other army - even with its mediocre units - IF the general knows how to use them right.
The one really painful lack in your force is chaff.
DE live and die by making sure they get the matchups THEY want, so you need to control the deployment AND the maneuver phase. This is done with lots of small, cheap units that drag out your deployment, forcing your opponent to drop
HIS main combatants before you have to drop yours.
Minimum-size units of Witches, Corsairs, Cold One Knights or Repeater Crossbowmen can do the job. But Harpies, Dark Riders, and Warlocks bring a level of maneuverability to your chaff that few other units can match; and this is what makes them useful and dangerous all out of proportion to their cost.
Shades are also quite useful in the maneuver phase, units of 5 with 2 hand weapons are good harrassers, diverters, war machine hunters, and general annoyances to the enemy.
But whatever you use - be it chaff, support, or main combatant - use it with care and precision to disrupt your enemy's maneuver, block charge lanes so your own frenzied troops stay in line, and in general to make the battle unfold to YOUR plan instead of your enemy's plan.
There is happiness for those who accept their fate 
There is GLORY for those who defy it 
