Send a mage into the room, ALONE, while your characters stand unseen outside the door. Speak with Torgal, and when he attacks, go invisible. Cos he is the only one who can sense invisibilty, he will attack alone. Lead him outside the room (the two trolls should just stand still) and have your party attack him. Then, when he's dead, finish off the other two inside the room, maybe by firing a cloudkill or two in there. Hope I helped.
PS: Welcome, Anthem!
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Horatio, Silver Ranger of the ALSB
Oh that lovely winged elf!
She's more valuable than the arrows of Melf,
More useful than the Scorcher and Magic Missle Conbined,
Join the ALSB, just sign on the line!