I finally completed dungeon challenge3.5 the only thing I changedI was I put a  hero with speed as leader maybe you have to cast before the boss and also maybe try raising your accuracy.  IHope this helpss

tjdroidaccnt - Thank you! I tried to finish this challenge several times with Tigersoul, but your comment helped! I put in a leader with a Speed aura, and another champion with a Speed Boost, and was able to activate the Block Debuffs buff on my champion before the boss cast the first Debuff. Yay!  

Raid support - maybe a description change on the Challenge is in order, indicating one has to do this before a Debuff is cast by Malek K.e (or whatever the actual rules are)? Just sayin... this one seems to have a few of us stumped.