[Winterous]

Blast weapons are probably the way to go if they're shooting units, they DS, shoot, and they're still clumped up, Plasma Cannon awaaaaay!
Against melee units though, they'll run after DSing so they won't be clumped up; Rough Riders are a good suggestion, a great unit, those, and generally massed firepower as well.

 

[Winterous]

Um, I hope he realises that Daemons (from the Daemons codex) cannot assault on the turn the Deep Strike.
Lesser Daemons from CSM can, but that's a special rule.