I found that the death line, specifically the spells Soulburn (requires fire 2) and Dirge of Cicera, to be incredibly powerul. The strategy is to corrupt all nodes in to death nodes, which provides a huge buff on these spells in combination with evoker and the starting soul staff. Dirge of Ceresa and about 7 death nodes with the impulsive trait does 70-80 damage to every unit on the first turn, and about half that each successive turn for 10 turns. This even works on the dragons in the final quest for an easy win. Soulburn does about 80-130 damage on a single unit, which is useful for those immune to poison.

I like powerful strategies, this one may be a bit over the top. Nearly all living opponents go down in round one with a net gain in mana from the soul staff (+3 per kill). In some battles I ended with up to a 80 mana more than I went in with. For instance, 8 units with 4 troops each, which is a large army for the AI, each get hit with 280-320 damage and everyone dies in round one. The profit was 84 mana (32 x 3 from soul staff - 12 to cast with some - tactical cost buffs). 

Even though this was OP, it was a lot of fun to use. I hope this isn't nerfed too bad.