Nirth: EDIT: I now made an honest try against him and I managed to get him to half of his health. The problem arose when he began to silence me and as soon as I picked up a potion to heal he made me spill it, I couldn't even press a hotkey fast enough to drink it as his action would interupt mine so I'm not sure what you're supposed to do at that state. Any tips would be appreciated.
Telika: Don't carry your potions. Scatter them around the room before the fight, and pick them when you need them. Don't try while you're under "mind control", pick/use them right after you hurt him, before he fires his beam at you.
Also, you don't need money to buy regen potions. Just go kill some satyres (if I remember well, you need their intestines) and go make a few potions of your own in the kitchen. You should have been able to buy the recipe by now.
I did but I was too nervous to do any safe micro-manage, I was more prepared for what he was supposed to do and in what order the second try which turned out to be a success.
As for regen potions, can they be bought as it's really annoying grinding Satyrs for their Intestines, it took a while to do that.