This boss has a fairly infamous reputation among the community already, for good reason. Status spam out the ass will ruin anyone's day. Diva's final boss will test your knowledge of all of the game's mechanics and the tactical decision making you'll have to do to bring it all together. Especially if you're a poor bastard like me who chose Diva as their first character, meaning you aren't carrying anything over from a prior most likely easier playthrough.
What setup am I using?
1) I used the Phoenix Dance formation and placed Diva in position 3. Phoenix Dance boosts the defense of the party member in position 3 along with increased enmity and stacking reduced BP cost for defending across multiple turns. The rest of the party members get antiair, which can help slightly with the minions in phase 3. No clue if the boss is also affected by antiair.
2) I gave Diva the Blade Attractor accessory and the Lightning Rod accessory, which greatly boosts Diva's chance of being targeted by slash and lightning attacks, which make up the majority of the boss' single target moveset. The former would have been picked up somewhere in Delta Base towards the beginning of the game, while the latter can be obtained from one of the red trials if your trial rank is high enough. Mine was also running Troublemaker (increased enmity) and Diva (50% mech paralyze resistance) roles. I also gave her other gear to boost her HP and resistances as high as possible, mine was at 550+ HP.
3) Since Constantine leaves the party for this, I had a robot as a replacement. I made sure both Diva and the robot had Guardian Program. I did grind a little bit to bring Guardian Program to Rank 2, which reduces the base BP cost from 3 to 2. Robots are immune to poison and petrification, so having both protecting other party members helps a lot in being able to attack and defend against the many AoE attacks the boss uses at the same time.
4) I made sure both Boudica and Blath had a parry option, purely because of the Emerald attack the boss in the last phase uses upon reaching 50% HP or less. This move will straight up end most runs if you don't have a defensive answer to it.
Beyond the formation setup and gear that I'm using, this party isn't really anything special. Everyone is using their default preferred builds from the beginning of the game, and the other robot I'm using is someone that is guaranteed to join during Diva's story. I would have loved to use Wednesday instead, except I used Constantine throughout the game and benched Wednesday from the start. While I did expect him to leave the party at some point, I thought it'd be a temporary thing that happens halfway through Diva's story and that he'd eventually rejoin, I legit didn't expect him to pull the backstab at the very end.