I like holy freeze merc with a poison necro to give you more time to cast poison nova, while enemies slowed. But if you have skeletons you cant use all corpses. I am telling you about sc, no idea how a poison necro works on hc.
I use clay+decre+this merc usually on bosses, and attract+low resist on normal monsters normally.
With a venomancer, go HF, as the slow effect only helps the timer on Poison. With a Bonemancer, I'd go HF as the slow also benefits your ability to cast AI curses and CE's. For a Bonemancer/Summoner hybrid like mine, I went with Might, as my skeles/SM has only a 10 point investment in each currently, and while they make for a pretty decen meatshield even at that low slvl, Might boosts their damage so they're also a decent killing mob.