KH Birth By Sleep Final Mix (1.5 + 2.5 ReMIX) - Final boss fight
6[WARNING: THIS VIDEO CONTAINS HEAVY SPOILERS FROM THE MAIN STORY OF KINGDOM HEARTS BIRTH BY SLEEP]
This video features the final boss fight of Kingdom Hearts Birth By Sleep where Aqua must battle against Terra after being possessed by Xehanort (a character usually referred as "Terra-Xehanort" or even "Terranort" by the fans). While the Final Mix version introduced in the game a Secret Episode (A Fragmentary Passage) with a true final boss (the Dark Hide), Terra-Xehanort is usually considered the very final boss fight due to the challenge level and the emotional climax of the whole thing.
Note that this video was recorded while playing Birth by Sleep on its Final Mix, 2.5 ReMIX version from the 1.5 + 2.5 ReMIX compilation for the Playstation 4 and on Standard difficulty. I was also on my Playstation 4 Pro.
The final boss fight against Terra-Xehanort is basically a reiteration of the fight you had with him already at the end of Terra's story. Therefore, the whole battle is as hard as you may expect it to be. To some, it's even worse than the first one because now you have to face the boss as Aqua instead of the Lingering Will and she's far more fagile than the latter and her counter-attack (the chief method to defeat Terra-Xehanort previously) don't do as much damage. Also, the fight is divided into two stages, so you'll have to completely deplete Terra-Xehanort health bar twice.
While the first stage doesn't differ at all from the one you fought as the Lingering Will (only he won't cast his Meteor desperation attack, which is nice), the second stage will be triggered by Terra-Xehanort summoning the same Guardian Heartless used by Ansem, Seeker of Darkness from the original Kingdom Hearts. This last leg of the fight is actually far easier than what you had to endure up to this point because the boss (except from an admittedly dangerous Ars Arcanum move every now and then) will relay exclusively on the Guardian, who will attack far more slowly and in fairly predictable patterns (most of his moves are taken straight from the first game, among them his desperation attack). In other words, it's safe to be a tiny bit more aggressive during this last stage and unleash your commands and finishers.
Towards the end of the fight the Guardian will try repeatedly to grab you in order to allow Terra-Xehanort to slowly walk towards Aqua and unleash Ars Arcanum. If you can resist the grab by winning the QTE you will be briefly teleported in an alternate dimension where you can reach Terra and unleash with his help a finisher that, if executed at the right time, will do a nice chunk of damage to the boss