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I don't know what other advice you've gotten, but what has always worked for me is to summon creatures to get up close and personal with Torgal & his buddies, while your party peppers them with spells and missiles. As the summoned creatures get killed, summon more to replace them.
If you've found any Darts of Wounding, try using them on Torgal; poison damage can kill trolls (I once took him out very quickly this way when he failed his save).
Good luck!
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