Level Loss

I particularly dislike level-loss mechanics, both as a player and a DM. My preference in implementing level-draining undead is for ability drain to be temporary (hours or days to recuperate), unless a key stat is drained to 0 — for instance, Constitution reaching 0 would mean death.

Alternatively, perhaps each successful attack by very powerful undead removes 250 XP from the PC's accumulated experience, not to fall below the minimum XP for the character's current level.