Well... I beat Kanji! First try!
I had to grind quite a bit though. Leveled my MC all the way to level 24. Did some solo training on 10F. It's easy to die but most enemies are weak to fire or ice so I made sure to have one persona with mabufu and one with maragi. Usually, I could beat all enemies with two of these skills.
However, since it's easy to die... I made sure to leave the dungeon every 2 levels so I could save and fuse new personas. Excluding a few hiccups and game over screens, I went from mid 10s to level 24 in no time. One of my personas then got that one skill where you regain 5SP every turn. I went back to castle 1F and I just kept using guard against the weak enemies so now I can basically farm SP without paying that fox
As for Kanji himself, I knew I'd need a persona with good physical defense because that seems to be important in this fight. I also decided to use the physical skills that make you lose HP so I can focus all SP on Media and just let Yukiko and Chie do ice & fire on the adds. Myself, I used this persona:
Not sure if this was a good choice. Went I dinged 24, he seemed like an okay choice because he was level 23. Also, he's strong against physical attacks and Brain Shake did like 180 damage if I was lucky.
What is important is that I'm finally getting the hang of combat. Fusing feels really natural now and changing personas during fights too. I'm actually really liking the combat system. It gets better and better. And thank god that stuff like poison does not carry over after a fight. I hate games that require you to have antidotes on you at all times just for that stuff