First time I fought BFA I didn't have secret aeons, celestial weapons, full over-drive bars. My Yuna was very bad.... That battle is HARD if your sphere grids are not full.
I would advise saving the "talk" option with Tidus until the second part of the battle. In the first part, BFA will use Triumphant Grasp, which isn't too bad.
A strategy I found very helpful was using a Stamina tablet on your character/s with the highest HP, doubling it. For me, this was Auron and Tidus, and so their HP was 9999. This way, BFA cannot kill those characters with Triumphant Grasp, or even Ultimate Jecht Shot (not positive with that). Also, use Zombie Attack (I actually had that with Auron, I had finished his Sphere Grid) so that the Yu Pagodas cannot heal him, only damage him. Quick attack can be helpful, though I didn't have that fighting him for the first time.