This is a problem I'm having as well. It's not just an issue of using fire/acid, or pausing to let him actually fall down into vegetable state, or even just his fast regeneration. Torgal will go down to Almost Dead status, but won't fall unconscious, no matter how much damage you do to him.
Now, I've seen some other forums discussing that it's an old bug dealing with the Bigger Picture mod (something to do with all trolls getting some sort of ring or amulet that conflicts with Torgal's script), however, I'm not using that mod. I am using Tactics, and have the Streamlined Trolls portion of that mod installed as well. There's a .cre file that you can substitute for Torgal's existing one that will make him killable (it also just makes him stand there and take all your hits without attacking after he says his lines). Not very sporting, but I've found no other way around this problem. It could be a conflict with some other mod (not that I can think of any that would mess with his script), as I've been able to kill him with Tactics installed before.