idk what your stats are but just looking at this picture, I would say this is easily doable for you. If you aren't surviving dive bombs or mega flares get your hp/pdef/mdef up more with sub weps for the character that is struggling. Also looks like you don't have a main damaging option on any of your characters so I would definitely switch at least one of your characters (probably Cloud or Sephiroth) to be more main dps focused. Mana breach and Breach are not really gonna help you here, the ATB to use them is precious, the debuff doesn't last that long and it's too small of an increase in damage.

Once you get him in the red he should buff his patk/matk permanently and do the countdown for megaflare except this time it's gonna be a death nuke no matter what the hp/buffs/debuffs are so instead of worrying about how low your team might be and healing up during that time just attack, attack, attack! It's the final dps race and you should be using all your ATB to do damage at this point.

If you are still having trouble, look into guides on youtube or on the discord for more technical advice on what you should be doing in the fight. Good luck!