Have to kill tier 5 boss with decrease def on him. killed him with it on him, some times hes had it on him twice in the fight, yet the mission doesn't complete. what am i doing wrong?

Double-check to see that you are going to the exact Stage specified in the Challenge description, that is usually the most common issue from what I've seen. But if you are certain that you followed instructions there to the letter, and the Challenge won't complete, a bug report with screenshots and player ID will be required here.