IIRC Farwil is the Dunmer son of the Count of Cheydinhaal that you help close Oblivion gates. He survived in my game without too much effort. If it's too irritating, turn down the difficulty slider, and switch to ranged weaponry to kill any enemy before he even gets close enough to get hit.. I can't think up a fix for making him run into lava..
Actually I can; but it involves a glitch. You need a lot of paintbrushes; like more than 50. Simple "drop" them one by one in a circle around him. If you don't have the unofficial patch, they should float. After you've completely trapped him, he won't be able to move despite trying. Then you should be free to close the Oblivion gate without him.
Bear in mind though; I don't know if this method causes future problems or glitches. Also there's a chance that if a Daedra finds him when you're not in the same cell, he'll be killed without you even knowing.. Try it, but it's not 100% guaranteed success.