Here's a crazy (?) idea: have the Priest heal weakening and corrosion on level-up. It fits with the RPG Priest "healer" style, isn't dependent on potions, isn't overpowering in easy dungeons but gives him an interesting niche in hard/vicious, and also directly solves the Slime Pit complaint, turning the weakening zombies into nice bonus XP fodder.
EDIT: Also, as far as "teaching" goes, encouraging the Priest to
become weakened/corroded by promising to cure it afterwards could help teach players (like me
) not to always fear and shun those status ailments as game-enders.