Pretty much what codeotaku said.
Use 3* Card % damage upgraded to max with 2 characters (I used Inoue and Lucia). Works great.
Use Inoue to clear status (it clear the boss buffs too).
Use one or two cards to heal. 30% HP single target was enough for me (though I used Shizuru card that deals damage and heals at 60% of its damage, from a previous event).
Also, revive items help a lot. Helios posted a screenshot of the item at April 14th, post 377. It’s a wooden thing.
You need to craft healings items before (the single green bottle), so you can use them to make the revive item.
Oh, and watch out his heals. When his HP is low you need to use other skill cards to finish him off quickly!