dorns deep

how can i kill the respawning lich in dorns deep

Not sure at which point you mean this. If you can't kill the lich at all, realize that you need +2 weapons to hit him. This means +2 missiles for ranged weapon users.

But he also regenerates after you kill him and will continue to do so. (The regenerated versions aren't worth any points, so you can't profit from this.) If you've been following the story, you know you have to take his phylactery into Jamoth's Tomb to kill him permanently. Both are located in the room after the one where you first fight the lich. That's the Hall of Heroes. The key you need is in one of the sarcophagi behind the spot where you first encounter the lich.

The lich will start regenerating in Hall of Heroes when you enter. I just put my three best melee fighters (with +2 weapons, of course) right next to the spot where he regenerates and had them hack at him whenever he appeared. Right after the regeneration he's pretty low on HP, easy to kill. The other three characters ran around and found the phylactery and took it to Jamoth's tomb.

Some tombs in the Hall of Heroes have statues in front identifying whose tomb it is. Jamoth's has such a statue. However, the lich's tomb with the phylactery is one of the unmarked ones. Beware that many of the tombs are trapped.