I've never had any problems with this quest before, but I'm at the part where you have fight the mummies and whatnot before the floor puzzle, and my current party is getting wrecked.
I've got Minsc at Lvl 11, front and center with Lilarcor and an AC of -5. I quickly got tired of Lilarcor, so added the mind flayer circlet so Cromwell could silence him somewhat, so he'snow a +5 weapon, and that's doing no good at all; Minsc is getting destroyed by the mummy without putting up much in the way of resistance.
The rest of my party (Alora Lvl 11, Viconia Lvl 11, and myself, a Lvl 9 mage) are of no help; nothing we do in the way of spells, arcane or divine, or anything else, does much to help. I've tried isolating the mummy in the hopes of improving my odds, but that doesn't help at all; Minsc still gets slaughtered, it just takes longer.