Use magic against him, he always follows the same pattern, so be sure to defend right before he attacks. If you have problems you can try to negate his ATK UP buff with Yosuke (hoping he won't target him). Physical attack are dangerous since he can counter them. Chie or MC having counter can also land some nice damage. If you can, buy some hawk eye, they give "dodge physical"
he has no weakness but has same "4 turn pattern" power charge, critical, boost strength, attack. If you dont have magic then attack for 3 turns then block with everybody after he does the third boost. you should take a decent but not fatal amount of damage.