Froze again at the second turn while buffing my team. I highly suspect the rain is the problem, as before the main freeze locking the game, there are 2-3 small freezes where rain doesn't fall but music is still ongoing. Usually if I lose focus, alt/tab, click away I don't have any problem, when I have one, the game simply close with a message, nothing similar. (I didn't lose focus during the fight) but it will only happen when a fight/scene is loading, nothing to do with the current problem ^^.
Also noticed something weird that I didn't see yesterday : Once Carmine Weapon is defeated, as soon as you go back to Kalm traveller (and without talking to him), there will be a materia on the ground (purple), labelled as "W-summon". Checking my materias, it's a underwater one with some AP on it.
Edit: The materia has 0 AP, the one with AP on it is the one I used during the fight (it gained AP as you forgot to set it to "Master" as someone mentioned before. I'll post screenshots in a spoiler to not waste a lot of space on the forum page.