After you beat the crap out of a troll it will fall down. then simply use some sort of fire or acid on it and it will die. though i have noticed that a spectral troll take more than one fire or acid arrow to kill it.
Poison can kill trolls too. As far as I know weapons that cause monsters to 'bleed' like valygar's katana can kill trolls.
Cloudkill also work on trolls.
I've noticed trolls die without going down (or immediately upon) when using such weapons - don't know if it is the onset of poison etc. afterwards or if damaging then with fire/acid 'pre-empts' their falling down.
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