I'll walk through the Lunar Subterrane again so that you don't get lost in the dungeon. Follow the map below to the final battle:
Boss: Zeromus
Immediately select "Item -> Crystal" and use it on the boss to reveal its true form.
This battle is just a matter of survival. If you cannot beat the boss, it means that your level is too low.
As long as you can tank the damage and are able to heal your party members without dying, you can easily defeat the boss.
This boss has powerful skills like Big Bang, Flare, Black Hole, and Meteor.
Congratulation!!! You've completed the game. Next, open your bestiary and see if you have any monsters left that you need to unlock.