What gives with this challenge? I had scale hacker from gromil decrease Def and vine trap from sorceress decrease Def both on the guardian. I still can't complete the challenge. 

And this is on the boss in Stage 5 of the Force Keep? can you take a screenshot of the challenge and also ensure you're meeting all requirements? There's no known bug with this.

Decrease def debuff icons present when the guardian is killed on level 5. Not sure how that doesn't meet the requirements? 

i

The next blow killed him. 

To clarify, the guardian was killed with the debuff, but the challenge is still not done. 

Been awhile since I've done that one, are there any other conditions in 2.1? Please post a screenshot of the mission, thanks.

i

I might be missing something here. I tried not to overthink it, which is what I normally default too. I tried a few different iterations of party composition. I even solo'd it with deacon Armstrong and timed it to kill the guardian with the debuff present. 

Ah, I see the issue. 

This is the wrong dungeon. you need to do the Force Keep.

Ah, guess I wasn't reading properly. Oops, lol

No worries, we've all been there. =)

Tried it 7 times, it didn't work. Then on the last round I clicked the info tab while in battle to view the debuff on each character while screen recording and it finally worked. 

So try to prove that it doesn't work and it will work....