My surefire tactic is fireball (to knock them off their feet), blizzard/inferno (cast while they're trying to get up), then that blood magic spell that holds them in place.

Leliana gets to pick off whatever is not dead right away, and if some do make it out of that clusterfuck, I've got two tanks playing cards right at the edge of it.

I decided not to take Wynne along because it would get boring. And sometimes I skip the aoe casts in favor of some arcane warrior melee hurt.

Loads of fun though.