Start off by having Chie use their mass target attacks (if they have them) to incapacitate his flunkies (keep them down, or the respective ice/fire guy'll attack Yukiko/Chie with their element or else put up an elemental wall) and concentrate on them before moving on to the main boss.
Have Yosuke guard when Shadow Kanji starts using magic, since he uses lightening magic (which Yosuke's weak to) and just keep hitting him with your best attacks until he goes down. --- Having to explain your joke doesn't make it clever, but having to have a joke explained to you doesn't make it unfunny.
Jack Frost is pretty useful in this fight. He can take down one of the side enemies with Boosted ice skills and he cures Chie and Yukiko's rage with Me Patra. --- "Once again, ViewtifulGene's logic blows minds and crushes dreams." -TheGamingGolfer "Supez teh king" -dedekong
Does he have any huge attacks I should be worried about, because I currently have him at ~50% health with only main character left, with obariyon for resisting normal attacks
He'll start using a fairly powerful lightning attack. Can't say I'd fancy your chances at holding out if its only the MC left as you'll likely be poisoned on top of being hit a lot. Your best bet would be just going back and trying to get Jack Frost like another said. He's really good for that fight. Any persona with Sukunda or Tarunda would help a lot also.