Hey dude, just saw this post and thought I would give you some pointers. I have been playing Scions since about a month after the book dropped and have noticed that there is not a lot of points of reference for people that are starting this army. While I am in no way an expert, I feel I have a rather good understanding of this codex's strengths and weaknesses, and thought I would throw in my two cents. Before I start I should probably let you know that, while I am not a competitive player, I do try to make powerful and efficient lists, and if anything I suggest seems too powerful, cheesey or just useless for the lists you will be playing in your area, feel free to not adopt it into your list.
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The first thing to be aware of is that these guys ARE NOT IMPERIAL GUARD. They should not be taken with 10 men, they should not be getting shot by commissars, and they should not be holding the line. These guys have some very big (TITAN SIZED!!!) weaknesses, but they also have some incredible strengths. I will list these.
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Weaknesses:
Easy to kill (A 4+ save means little)
Weak Vehicles (A Taurox can be glanced to death by bolters)
Small Numbers (No platoons or 20 man units)
Strength 3 Guns (Those AP3 Hellguns need 5+ to wound SPEHS MEHREHNS!!!)
No Amazing Units (You cant rely on any one unit to win you the game)
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Strengths:
Mobility (Fast Tanks, Move Through Cover, Deep Strike)
Cheap in low numbers (A fully upgraded 5 man squad is cheaper than an bare bones 10 man!)
Overwhelming Fire Power( 2 Special Weapons per troop squad)
Versatile Vehicles (Taurox Primes can be turned into baby versions of Baal Predators, Whirlwinds, Russ Tanks)
No Amazing Units ( What can your opponent shoot that you dont have 10 more of?)
Battlebrothers with Imperium (access to Inquisitors and Assassins)
One of the best battleforged armies ( Objective Secured on Taurox Primes that can move 18" to not contest but SECURE!!!)
Only Troops and
HQ (Easy to fulfill minimum detachment requirements)
When making your list, keep these in mind. Now I will go over all of the units. When picking these, think in terms of detachments.Each detachment should have 2 command squads and 2 Scion Squads. This way your scion squads each get an order every singe turn. These detachments should be kept with the same weapon loadouts, so have one with all meltas, one wit all plasmas, etc. and then group them (in their Taurox) with other detachments.
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Lord Commissar
Unless taken in a formation he is useless, and even in the formations he is useless. Due to small squad sizes, he is more hurtful than helpful, and for the price of one of these guys you can get another scion squad.
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Command Squad
Epic in every sense of the word. Give these guys meltas, Plasmas, or Volleys, based on what their scion detachment will use. Take 2 of these per detachments to give out max orders per turn.
While you may be tempted to give them 4 special weapons, it is a better option to give them a Vox (make sure your orders go off) and a med pack (this will win you games). If you give your squad plasma guns, give the Tempestor Prime a plasma pistol, otherwise a bolt pistol or Hell Pistol will be fine.
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Scion Squad
The Bravo Team to your command squad's alpha. Take these as 5 man units with 2 special weapons (Melta, plasma, or volley) and give them a Vox to make sure orders work. As with the command squad, give the sarge a plasma pistol if his men are getting plasma. A "Plasma Detachment" should be your main force due to its anti infantry and anti AV12-13 abilities, with a Melta detachment for dealing with AV13-14.
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Valkery
It is gak, dont waste your points or money. Not as good as a Vendetta. A HUGE part of this army is making the things your opponent payed points for utterly useless. If 20% of their guns have sky fire, and you have a flyer, your flyer is dead. If you have no flyer, 20% of their army is rendered nearly useless.
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Taurox Prime
A bit costly, but the cheap price of your scions make up for it. DO NOT, I repeat DO NOT give this thing a Gatling Gun. 10 S4 shots with
ap- is worse than a tactical squad shooting. For anti Infantry take the standard Russ cannon, and for everything else take the Missile Launcher. This lets you play with baby Russ tanks and baby Whirlwinds. The Missiles let you fight med to heavy tanks and light infantry while the other one lets you fight all infantry and light tanks.
Id suggest a mix of the two. Regardless of main gun, they should ALWAYS have autocannons (These things are your main long range anti tank guns) and should also ALWAYS have Camo Netting. A av11/10/10 tanks is as weak as paper, but suddenly becomes "durable" when it can sit in the open and have a 1/6 chance to dismiss any shot at it. If it is obscured (25%) It gets a 4+ instead of a 5+ due to the net, and behind an Aegis defense line it gets an epic 3+ Cover Save!!! WOW!!!!!
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Allies
There are quite a few good allies to take, but the best are by far Inquisitors with Servo Skulls (This allows you to manipulate where your opponent moves and can help give you accurate deep strikes if circumstances call for it.) and Vindicare Assassins. The Assassins are by far my favorite as he can pick out a single model and basically declare it dead. A 2+ to hit with a reroll on a 1, a 2+ to wound, and 72" range on an AP2 gun is insane. Top that off with amo that allows for
Str 10+
d6 vs tanks and you have an even more glorified version of Chris Kyle dressed like a ninja!!! For inquisitorial retinues, a full group of Psykers is nice, because it gives you free warpcharge dice for the psychic phase. Add those to your inquisitor and suddenly you have a good handful of dice for an army without psykers. Imperial Knights are also really nice as you can fit 2 of them in a 2000 point army, and they take a lot of the heat off of your Scions.
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Building the army:
Build your army with a toolbox mindset. Have enough things that you can adopt any strategy when you play. Either deepstrike melta suicide squads to take out high value targets, or maybe use hunter killer missiles to weaken your opponent turn one, before they get a chance to hit you. Remember that your units are all glass hammers, and while they can hit extremly hard, they cant take it back, and should not try. Always send in a scion squad with the thought that they will die. Do not be affraid to retreat, it is better to give an objective mid game and then reclaim it at the end, then hold an objective all game and not have enough units left over to make a final push. Your speed, and objective secured, as well as your quality, not quantity, or fire power, is what will win you games. I really hope this helped and has given you some insight on how to get the full potential of this army.
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If there is anything you disagree with, or have questions on, let me know, This is one of my favorite armies
GW has produced in a long time, and any discussion about them I find enjoyable.