This occurs after you finish the Chapter 6 of the main game and earn
You go on a series of missions to let you upgrade to Class 4 then can begin the final quest. The dungeon is lengthy and there's a very obvious Gold Save right before the battle.
Go ahead and grind out levels on Easy. You can play through the game on any difficulty and then just change the difficulty to Hard right before the fight.
To change difficulty press to bring up the character screen then to access Pause Menu, then choose Options and Difficulty is the third option down.My main was Duran with Angela using offensive magic and Charlotte healing and curing status aliments. I beat the fight first try using the exact same tactics I'd employed the entire game which was to rush in and beat the crap out of it. The only thing to be aware of is it can inflict various ailments like Moogle or Petrify so keep those cure items handy.