If you have the quest line (after finding a way into the sieged castle) and then make it to poison the Briarheart tree (can't remember quest name) then you are praying for no-one else running this quest with you... trust me.
Basically - I've run this quest several times now on different characters. First time I flew through it (did it on the first day I think the DLC came out). Second time - not so much quickly. Took me about 45 minutes to just kill the tree. My hands were cramped up by the end of it. You see, there were about 5 or 6 of us there running the same quest - which usually makes the quests faster and easier - but this made it INSANELY tough... because the spawn rate of the tree menders was obviously connected to the amount of people running the quest... we had a couple of low level players with us that didn't really know what was going on (we weren't in group or anything - and they weren't in area chat to talk to them). So... 3 or 4 of us were killing the menders, but they were spawning faster than we could deal with them. I actually broke a controller in frustration and button mashing from constantly being rooted (due to the amount of adds there were).
FIX IT!!.... I gave up third time trying to run it - and am now getting up at 5:00am to see if it is empty on that quest so I can get it done. I shouldn't have to do that...