Well, I would definitely suggest removing the ability score draining. But if you used it already, that's kind of limb solution. Reduce its number of hit points. Reduce its attack bonus so it doesn't hit as often.
Your players may consider focusing on the limitations his undead state brings him. (I haven't played or DMed the adventure, so I'm assuming it uses the ghost template.) For example, searing light does 10d6 points of damage now against it, no save. Sure you have to make a ranged touch, but dragon touch ACs are not tough. Combine that with a haste spell and its 20d6 a round.
The players could put a wall of force or a wall of stone over the acid pool to give them some firm ground to fight on. Or lure it out of the pool in some fashion (if that's possible).
Any of the elemental resistance spells could be userful against the acid as well. Or you could tone down the acid damage the pool does.
I tend to think of solutions as a player rather than a DM, so maybe that's not what you are looking for.
Your players may consider focusing on the limitations his undead state brings him. (I haven't played or DMed the adventure, so I'm assuming it uses the ghost template.) For example, searing light does 10d6 points of damage now against it, no save. Sure you have to make a ranged touch, but dragon touch ACs are not tough. Combine that with a haste spell and its 20d6 a round.
The players could put a wall of force or a wall of stone over the acid pool to give them some firm ground to fight on. Or lure it out of the pool in some fashion (if that's possible).
Any of the elemental resistance spells could be userful against the acid as well. Or you could tone down the acid damage the pool does.
I tend to think of solutions as a player rather than a DM, so maybe that's not what you are looking for.