Actually, I always go around it so I enter from the south-west. I then lure the troll out, and shut the door (the supposedly intelligent mage can't open a door...what is the world coming to?). I then deal with the troll around the corner, as the mage can still cast spells through the door (well, only one). After that, I kill the mage, as it's protections should have run out.
__________________
|