I have never fired wands or spells into the 'fog of war' for this battle.
My tactic is:
1. Cast Haste, Bless, Chant and any stoneskin spells available on my party
2. Edge forward with a character in stealth mode and lure Torgal out by himself
3. Have my best fighter (equipped with health potions) engage him in melee combat while the others use their best missiles/arrows/bolts or acid & fire arrow spells.
This way I don't have many problems and I ALWAYS do this quest straight after coming out of Irenicus' dungeon (at least as soon as I have picked up some NPCs!).
Caleb is right, the most important tactic is to concentrate on 1 enemy at a time as the regeneration rate is astounding.
The other important point is to use the Haste spell - it speeds up your attacks and this is really the only sure way of beating Torgal's regeneration if you engage him in straight combat.
Good luck! It can be done at any level, if one strategy doesn't work, try something different!
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Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas