This Prick, Moebius, from Time and Eternity.
In classic JRPG fashion Moebius has 6 phases for no adequately explained reason, including three of the game's total 4 fights using a scrappy mechanic to really up the annoyance factor.
Now for the actual fight, after running through 5 lengthy, but fairly easy phases, you get to the sixth phase, where Moebius turns the game's time spell mechanic (which you were using the entire game) against you. Using it to either greatly enhance their speed, reset themselves to the start of the fight (read full heal) or to freeze you in place for several seconds. All chosen at random so they have the ability to heal themselves, to full, however often they feel like, and done at the end of like half an hour of combat and cutscenes.
Then, they are also the only boss in the entire game to have a cinematic attack triggered when they reach half health that is likely to do 4/5ths of your health, which naturally has no saving throws. So if you're injured at all when you knock them to half you're very likely to die. In the 6th phase of a lengthy boss fight, with no chance to save in between.
The fight also starts maybe 5 minutes after you beat the boss of arc 3, so if you didn't save immediately after fighting them, you've effectively got a seventh phase to run through all to get to the only part of the game that's even remotely challenging.