Ah - some options I have seen used :
1) If Patch installed and you have Zenmaster, go out, wander about on own, gain experience, get lavawalk
2) No patch - stuck in tomb - lot longer wander on respawn high to gain enough experience.
3) This one has been successful I hear from others - jump to the closest stone to door and heal party - take party healer - cast many many many many (and then a few more) toughen spells and raise HP to 300 plus - (save game) run party across to door taking loads of damage - unlock the door using the two keys - if you are quick, you have about 20 HP left - now use all those resurrect spells (scolls if you have them) on all your dead party members, wait as long as it takes to get everyone healed up and your mana back up to maximum - time to wander down the passageway then and give Cet a good kicking
.
Good Luck
Davros