Pielorinho
Iron Fist of Pelor
Another approach: assuming that the party is mid-level, turn into a small child. When the party enters, speak to them formally and with a cold smile. Try to creep them into leaving: after all, what the heck's a kid doing down here?
A similar trick might work with turning into an undead-shaped humanoid (although this is technically beyond the ability's power, it takes very little tweaking to say that this special phasm can manage the appearance of being undead). Turn into a mummy, and they might avoid fighting up-close; turn into a skeleton, and they'll not fire arrows at you. If you're lucky, the cleric will cast Cure spells on you! (Or might just try to turn you, wasting an action).
Daniel
A similar trick might work with turning into an undead-shaped humanoid (although this is technically beyond the ability's power, it takes very little tweaking to say that this special phasm can manage the appearance of being undead). Turn into a mummy, and they might avoid fighting up-close; turn into a skeleton, and they'll not fire arrows at you. If you're lucky, the cleric will cast Cure spells on you! (Or might just try to turn you, wasting an action).
Daniel