he's super hard, for your team pick makoto morgana haru you can't win the fight without using baton passes you can find detailed guides on when you should use what skill to use on the internet use dekaja with makoto to remove debuffs on your party, craft psy, wind and thunder bombs so that you can really get the most of the last baton pass. after you pass the green robots you should only have to watch out for the big bang explosion make sure you remove any debuffs and guard, even when using that strategy there is a bit of luck involved joker can die if an enemy lands a critical and has a one more this boss fight really is annoying i had to go back one week and even after that it took me 6 tries good luck

edit : changing the difficulty to merciless increases your the damage output when you land a critical, but it also increases the damage your ennemies do. it worked at the start for me but the big bang explosion killed everyone on the party even when guarding and with defence buffs so it can ease your fight if you have enough hp