first be sure to have the spell physical shield it helps alot. so does slow time. go into greatwood caves map and kill the troll that spawns. then run into hobbe cave and kill hobbs until your multiplyer is at about 20 when your back in greatwood cave map kill the troll again. then quickly run and talk to the door,
make sure you have shield spell on n good luck
by the way you only need 14 multiplier if i am corrct and the troll only spawns after a certain part in the game not sure when