Let's begin with why you take tempestus scions, cmd squads, primes, and valkyres. Scions first, they are amazing in groups of 5 to 7. Now I say this because they get 2 special weapons no matter how many guys are in a squad. They are all around good hunter with BS 4 and 4+ armor. So long as you stick to your battle plan for the squad they will serve well (aka don't use the plasma gun to take on say a land raider or a high toughness creature. (We'll get to that second one later).
Now lets talk cmd squads. They are the orders of the turn so you want 2. Each can take 4 special weapons, I suggest the hot shot volly guns here as they have a 24 inch range and these guys deep strike. At the mid field you attain control of the board and a good area for order issuing. They are your marine killers.
WAIT WAIT WAIT! What about transports and objectives and flyers. Oh my....that his where we fight vehicles with vehicles. We have the taroux prime a fast transport with enough room for a fireing point on each side. They can flat out 18 inches, and are dedicated transports for your scions, making them ob-sec. And we have quite the selection of fire power on these ba he's fro. Mid range assault to transport poppers at the 48 inch range to antihoard at 24 inches.
For flyers we have Valkyrie squads. These angels have quite a fun selection. Of wonderful explosion initiators for your very best Michael bay impression and our scions can grav-shute from them for better positioning and reinforcement.
So what I want to know is, and I know people use this Dex mainly for allied detachments, what's your favorite u it and composition ? Why, and what drew you to it for this Dex.