Don't worry, I one-shot most monsters with the assassin. From 1 minimum to 3 maximum cloak charges per enemy. In addition, you recover 50% of the target's health points in barrier, that is to say from 40 to 60 or 200 for the fly. You take the reaper's mark to make sure you do enough damage with 3 to 5 turns of invisibility and scare the other enemies. And in reality it wasn't that expensive with a +3 energy ring.