Complete x dungeon challenges are triggered by killing the final boss. If the boss was already dead or something like that and you just got to the end, it will not trigger.
Alternatively, spawning in the boss in admin mode and killing them will also count.
Also, fix your sword, that thing is about to break!
If admin-killing, I’ve found that area kill works, directly killing does not (Crtl+shift+delete to kill radius, vs just ctrl+delete while targeted on the actual thing).
This unfortunately doesn’t work either =<, Waiiting on the video on manually killing it too upload but here are both Admin kills: Imgur Link for Admin Killing
Edit:
Sped up because imgur doesn’t like videos over 60 seconds.
Similar issue on a modded private server running Exiled Lands. We are getting Legendary tasks to complete until it is a boss in a dungeon. I had a player encounter this with Legendary Complete the Den of the Scorpion Queen.