Upon first entering Guhkbar's Pit you'll need to immediately need to do something for light. Once you have some light go down the passage to the west. When you see something in front of you choose the option to wait. A sprite will talk to you and give you a quest to free her mistress, Illyree, who Guhkbar is holding captive.
Follow the sprite through the rest wall and you'll find yourself in a passage leading to the south. You'll have to fight quite a few bats along here. Shortly after the passage turns to the east you'll come across a drop. The only way into Guhkbar's Pit is by taking this drop, but be aware that it's going to damage your characters. You'll want to heal them right away. Now you have two options of which way to go. If you go to the north you'll find yourself in a room with a pool of water and some Healing Herbs lying on the ground. Right around the corner to the east is Guhkbar. If you're going to kill him yourself, then it's best to go into combat mode before going around the corner, and cast Haste and War Blessing on your characters as well.
Head around the corner and kill Guhkbar. Go over to the eastern side of the room. To the south is a single chest, which contains the key to Illyree's cell. There's two chests to the north, both of which are trapped. You'll need a Tool Use of 12 to disarm them.
The alternative method is to head north after the drop. In the northwest corner is a room with some Cave Slimes. The next passage over leads to Illyree's cell, and she'll tell you how you could sneak to the chest with the key to her cell without having to fight Guhkbar. Using this method you can retrieve the key and free Illyree without the battle, but Illyree won't give you as good a reward, and she'll kill Guhkbar herself, meaning you won't get the Experience.
The northeastern corner of the pit contains a Worg and a couple wolves.
Once you've done everything, head out through the portcullis in the room where Guhkbar was.