So last night, I finally defeated Duncan. Third times a charm as they say. Of course, I had help so thank to my three friends who helped out.
One warrior (me), 2 eles (1 human 1 hero), 2 monks (1 human 1 hero) , 1 human ritualist, and 2 hero rangers. Our human ele was running endless meteor shower, the hero ele was running fire aoe spammer, the ritualist was spirit spam, 1 hero ranger was running burning arrow with apply poison, the second hero ranger was running broad head arrow, the hero monk was running LoD, and the human monk was running bonder. I was running a crazy build with Defy Pain as my elite (full build below).
We used an Armor of Salvation, an Essence of Celerity, a Grail of Might, and a Powerstone of Courage. We overdid it just a bit but it worked wonders.