This level is pretty complicated, but here is some instructions: Collect all the cell keys in this level. Cells 1-6 only have enemies, but I would recommend fighiting them for the experience, which is useful. Some of the cell keys are hidden behind false walls, but using the above map should allow you to find them. Open all the cells in order, and in the 7th you will find the key for the eighth cell. There you will find Prince Storn Stoneshield, very grateful to you for saving him. Now you need to return to level five, just trust me, you'll get a special gift from the dwarves. Fortunately, you don't have to walk all the way there. Go back to the start of this level, return to level 7, and the nearest teleporter will warp you to level 5. In level 5, go to the Dwarf camp, talk to everyone, sell items, etc... and then return to level 8 with that teleport I told you to remember about. You will be teleported near the exit of the level and near the crystal key. You don't even need it to exit the level after all! Pick it still, and then leave the level.
The enemies in this game are finally showing some variation! This level has several Drows, Hellbeasts, Rust Monsters, and Displacer Beasts. Rust Monsters are of specific concern, because their magic spell will destroy metal armor (I learned this the hard way, losing a full plate +2). The enemies in general should be easy for your party, which should be around level 6, if not at level 7. If they aren't this high, you should still be okay.