I can't defeat Torgal with fire after downing him

Casters

I've tried using arrows of fire, acid arrows, melf's acid arrow and flame arrow (spell); but I keep getting the message that he's resistant to magic. I currently have him downed on the floor, what do I do?

AionZ

You get a 'Magic Resistance' message? Try smacking him with the Flail of Ages. As I recall, the elemental damage on it goes through MR.

Casters

Oh right, I do remember that weapon mentioned at the beginning of the quest, and found the hidden forge for it. Not sure if I passed it though.

Grond0

There's an axe in one of the containers in the golem room that does acid damage that will also kill TorGal - though he's only partially magic resistant anyway, so spell attempts will get through if you keep trying.