For a warlock, I'd go with simplicity.
Some variation of the 0/41/30 build.
Fel Synergy 2/2, Demonic Knowledge 3/3.

Go with straight shadow bolt or immolate/incinerate spam.
You're not quite as good damage as a solo lock, but you're not far off of the mark either.

You have Seed of Corruption for very strong AoE.
Decent single target dps.
Soul Stone, Health Stones etc.
Curse of choice.