I suggest trying the instructions Rexaar had posted, and if you cannot find a way to continue the quest, submitting a ticket would be the next best option!
Here's the link to Nexon's Customer Support page:
https://support-maplestory.nexon.net/hc…
There currently seems to be an issue with the completion of that requirement, it's been reported in the Bugs section as well!
https://forums.maplestory.nexon.net/discussion/29513/100-mana-battle-objective-cannot-be-completed#latest
Hi!
This is an issue related to how long your computer has been 'running'.
Please see this thread for instructions on how to fix the issue: https://forums.maplestory.nexon.net/discussion/151/clock-overrun-was-memory-dll-corruption
You'll have…
Have you submitted a report/ticket via Nexon's Customer Support site, or just the in-game report option?
Here's their Customer Support page:
https://support-maplestory.nexon.net/hc/en-us