Fighting Shadow Kamoshida requires multiple steps. First, you’ll have to waste a bit of your time attacking him. After about two turns, he’ll heal, providing you a new target: the Trophy of Obsession. It’s like a large goblet filled with female bodies (presumably the bodies of female students that he’s been coveting). It allows him to completely heal all damage, so left unchecked, this fight will be endless. Simply have everyone one of your characters target it, but every three turns, have Ann use Tarunda to keep the main body’s attack power down. Don’t worry, the Trophy itself doesn’t attack and Kamoshida won’t bother to use it as long as you don’t attack him.
Another thing that happens after the boss heals is that you’re informed of a new mechanic in Persona 5 – Special Orders. This allows you to send any one of your party members to do something separate from the team. It makes your attack power weaker, since you’re removing a member from your party, but if it works, you can win the fight easily. In the case of Shadow Kamoshida, you can send someone to knock his crown off, which also happens to be the treasure you were trying to steal. You can choose anyone, but you should choose Ryuji. This way you can keep Morgana healing and have Ann weaken the boss as you fight.
All you really need to do is take out the Trophy of Obesession and survive for around five turns. If you manage that, then Ryuji will knock off the Crown, greatly weakening the boss. Now all of your attacks will do a few hundred points of damage. Use your strongest Persona abilities and you can end the fight in a single round.
Ann won’t kill him so that the real Kamoshida can confess his sins in the real world. You’ve just beat your first major boss in Persona 5 and the Phantom Thieves are on the rise.
