Duncan the Black
Does Pain Inverter go into a feedback loop, instantly killing the person who cast it the moment either take damage? i.e. You cast pain inverter on Duncan and deal 60 damage, you take 60 damage from Duncan's defense. He takes 60 damage from pain inverter, and he does 60 damage with Duncan's Defense, repeat until you are dead.
If so, would that make piling enchants on someone and using symbiosis to make that someone have 1000+ hit points and then using them as a 'bomb' with Pain Inverter viable? You could bomb him for 1000+ damage every 20 seconds (keep everyone outside of spell range, rebirth the sacrificial lamb, reapply enchants, pain invert recharges and bomb him again)
Alternately, Flat and Longbows have an attack range that's longer than spell range, so couldn't you just load up a ranger hero with Apply Poison and Hunter's shot, set them up outside of spell range (since he doesn't move), and go take a nap/read a novel while degen (slowly) kills Ducan and your healers only have to worry about healing the ranger from puny amounts of retributive damage (everyone knows rangers can't DPS with a bow)?
I've killed Duncan once (broadhead FTW!), and I'm wondering if either technique would work to make him go down faster/easier.