Witch - Finesse, Int, Mem
Necro, Summoning, Bartering,
Conjure Incarnate, Mosquito swarm, Decaying touch
racial talent - Leech, Savage Sortilege
Personally I would probably take Blood Rain over Decaying Touch, as it seems like it would have better synergy with Leech and Conjure Incarnate, since it would allow you to have an easy way to heal yourself with Leech and summon a blood incarnate (to work on lowering armor for necromancy effects).
I hadn't thought about using Summoning with Necromancy before. That's actually a pretty cool combo. I imagine a water incarnate's additional restoration combined with decaying touch would be particularly effective.
