Last night I went to Aleroth and helped Zandalor close some portals. You have to keep the creatures from hitting him so that he can complete the spell. If he gets too much damage he has to start all over again.
Wave after wave of monsters keep comming out for you to kill. Tip - take your time and let Zandalor start his spell over again....and again. Meanwhile, you keep killing creatures and getting exp. Eventually you will see the experience go to zero after killing creatures.
THAT'S when you finally fend them off for good. I actually went up almost 3 levels when I did this. I was lev 29 when I did this though. I would not advise lower levels to try that thinking they will make it out.
JUST A HELPFULL HINT FROM YOUR OL' UNCLE BOOGER
