@Littiz: no no, no mods or improved-somethings involved methinks....
Mekrath is just the loser who used to hold Haer'Dalis captive!
(Ok, ok ... maybe there's an Improved Mekrath mod around that I'm not aware of but....)
My guess is that the problem is, you're not suppsed to attack him... You should do the quest in the non-violent way... Because you are so low level and he's suuuuuuuuuuuch a powerful wizard...
So, if you choose to attck him, he reacts as the standard Bioware's very very powerful mages that you are not supposed to be able to defeat (Irenicus comes to mind)...
Namely, he cheats
(And still manages to lose as well)