I have tried with two different champions (Marked and Fleshmonger) several times. Both have landed a decrease defense debuff on the Stage 5 Force Dungeon boss but when I kill it, the challenge doesn't complete. I tried going back to stage 4, debuffing that boss, then continuing with stage 5. That didn't work. I also tried force closing the app and then repeating, and that didn't work either. 

I had already beaten the level before, and the concensus seems to be that that might have caused the problem, but I've tried several times since then to pass this challenge, to no avail.

Should I reset my account/start a new account? I'm a completionist and it's really discouraging to me to think that this will forever be stuck.