How do you see your typical 3 or 4 mob trash pulls going?
How do you see your boss fights going?

Are you thinking of DoTing up all the mobs being tanked in which case you need really good planning to deal with runners?

Maybe DoT a single target up, Lifetap and then Drain Life combo to be as mana efficient as possible on trash. For bosses DoTs and Shadowbolt spam with Lifetap and Drain Life when low on mana.

3x warlock Hellfire will be insane.

Also, be careful with CoA, it is a waste of mana on trash usually.