1) ~175 resist is enough (more resits -> more heal)
2) there is no heal if i change weapon to bow
3) no heal on melee attacks, ranged only (like imps)
4) 8 imps at The Deep Cauldron give ~20hp/sec (266 resist)
5) it's possible to get overheal like 20000HP
Looks like it's bug of "Shield Expert" skill, like "an extra +2 vs projectiles" goes to heal.
PS: Hm, if i kill The Sleeper with 20000HP will I survive? Should try next time...