I personally think Skolas was the most challenging boss to take down. He was a definite pain in the rear especially during Arc & Void burn, Angry, Airborne and F#cking Lightswitch challenge modifications. Quite a few people are posting why Skolas is leading the votes when you can solo it now. I should have mentioned, "at that period of time" when the Boss was current which one was the toughest. I can tell you this right now there were not a lot of people light level 30-32 who soloed Skolas in year one.