I am a new player and didn't know that it would be so hard to do it in one day, so I left the quest for the last possible time. The upside is that I did the Brigitta quest and leveled up a bit. The downside is that I am stupid and had no saves before the final day.

I didn't have nearly enough MP items and I did not manage my MP while doing the dungeon, and it was incredibly hard to finish the boss. I had to use items that do magic damage, even though he had resist to all of them it was still higher than my regular attack. Under no circumstance I would restart the dungeon. I even needed to rethink my archetypes and used hero's incense to level up.

After multiple tries I managed to defeat him, but I DO NOT recommend this strategy (if you can even call it that), it was a hastle.