. . . the area of The Pit accessed from the Hall of the Pit fairly safely. Using ranged attacks, and staying within reach of the door, concentrate on one Queen of the Dead. If you can kill it without suffering terminal damage, your party can clear the area, because it can go back into the Hall and rest after each kill. Dispose of the Queens of the Dead and the Power Liches first; only kill anything else if it gets in the way or attacks you.
Staying within reach of the door is key, because you can suffer a lot of damage very quickly in that place, and you need to be able to turn and exit almost instantly when one of your characters gets perilously low on hit points. Losing a character in that place is likely to be fatal for the rest of the party if it hangs around in there.