Did someone managed to beat Grurn? (http://www.uesp.net/wiki/Morrowind:Siege_at_Firemoth_%28quest%29)
It has an insane regeneration health rate, and it's bugged. Once it doesn't have anymore magicka, it attacks you with a staff, never touching nor dealing damage, but even with that, i couldn't beat it with daedric weapon...

Grurn is supposed to be
Grurn is supposed to be really tough, your followers aren't expected to survive. However I did leave one wrong spell in there, this fixes that. I also removed his staff but he will only use spells now, the original used a sword but Oblivion Liches can't use them.
To resolve this I'll add skeleton animations to the Morroblivion Lich in this file. They will now use their swords. The other possibility would be to have them use the entire Skeleton skeleton with their animations and get a casting animation from somwhere else. Morrowind Liches where more like skeletons anyway.
It probably only works if you reload a savegame from before you first met him. Also what level are you? My lvl 48 character was able to deafeat him in less than a minute, at least with this file, this official mod came out after the game so it was aimed at higher levels.
Note that this is somewhat experimental and you don't have to use the animation files, maybe you even shouldn't. But it seems to work fine.