Can't speak to Phoenix, but I can say that with Mother Talzin leading Nightsisters, you can do this so long as your Old Daka survives the AoE. Just spam special abilities to keep stacking Plagues on Sidious (Have Old Daka use Celerity, since she only has one other special and you don't want to risk it being on cooldown) and he'll drop at the start of his second turn. There's a high possibility that you'll still have dead characters going into p5, so remember that Night Blade Assault will res all dead allies if it finishes off an enemy--the Deathtrooper in p5 doesn't have a lot of health, and you need him dead anyway, so...
