The way to tackle it is as follows. Save your position just outside the Hall of the Pit. Enter the Pit, and despite any temptation, don't go more than one move from the door back into the Hall. You can suffer a lot of damage in a very short time in that place, and you need to be able to get out quickly. The dangerous characters are the Power Liches and the Queens of the Dead. Basically, what you need to do is to get rid of those one by one, resting between kills.

Pick on one QotD and try and Paralyse her. If you can kill her using ranged attacks you can clear the Pit. When your party gets damaged, escape back into the Hall, and Rest. Don't try and kill more than one QotD at a time, because the rest of them will by then have Pain Reflection up, and the reflected damage will hurt your party badly. If there are any Power Liches close to you, you may have to kill them first, but the Queens are the more difficult for a party at that level.

If you want to tackle Nighon instead, you need to know how to get out from Mount Thunderfist quickly. The quickest and easiest route is to bear NE for a short distance then go west and follow the tunnel south to the 4-way intersection. From here head NW and there's an exit a little way along there. I wouldn't go there, though, until you have a sorcerer who is ready to learn Town Portal after visiting Tobren Rainshield in Nighon Town to get Mastery in Water. You don't really want to tackle the tunnels to Nighon twice!

Good luck, and HTH!