What, you mean there are things out there that party's can't automatically waffle-stomp? Egads!!!
Actually, a demi-lich is very little trouble for a party of 15th level character. Or even a party of 11th level characters if their cleric has a bead of kharma. The reason is that the thing has so few hitpoints and such a miniscule AC that any fighter worth his salt should be able to deal 52 some-odd damage firly rapidly with the +5 weapon the cleric gives him.
Looking over the things that can harm it (note taht the Conversion has been toned down a bit, these are the originals):
Fighter-types with powerful weapons (easily gained via a friendly Cleric). Heck, if there is a Paladin in the group, there's a sword lying on the floor that can do normal damage to the thing. - normal damage
Dispel Evil is a pretty nice spell in its own right in an area where undead roam. - 5 damage
Holy Word should be a staple in any high elvel cleric's diet. - 20 damage
There is 16 points of damage available just lying on the floor. Granted, a thief (or anyone for that matter) will likely not elect to hurl a massive gem at the thing.
Shatter is actually a very useful spell when exploring an area rife with traps. If the mage thinks to cast it on the skull - 10 damage.
As for finding out the thing's weaknesses, maybe the party should flee after the first time they are stomped, then use Divination, Legend Lore, etc. to learn things about the creature? Then again, maybe I'm wrong and high level parties shouldn't be able to find things out like that.
