Tell me about some bosses you remember that had 3 phases.
Been going through Cathedral and that final fight has a Phase 3....
I haven't beaten it yet but still.
How bout 4? With an additional side quest to defeat 8 mini-bosses to regain your abilities that are automatically locked prior to the fight.
In addition, leveling up is bad because the boss' scaling triples the difficulty if you foolishly max out the main character's stats.
And then to make matters worse, the longer the fight takes the more abilities you lose at random.
If a single party member gets knocked down they are immediately removed from the fight permanently and summons are insta killed.
That boss' name is Ultimicea.