Ultima is one of the toughest challenges in Final Fantasy 16, and that goes double on Final Fantasy Mode – but players can make it easier by leveling up. Final Fantasy Mode is the hardest difficulty available in main story gameplay. It only becomes available on New Game+, which players can unlock the first time they complete the game in Story-Focused or Action-Focused Mode. In order to provide a greater challenge, Final Fantasy Mode levels up most enemies, gives some of them new patterns of attack, and adds more of them at crucial choke points.
https://screenrant.com/ff16-final-fantasy-mode-ultima-level-clive/