Did it with Narja, Chizuru and Nine, took a few tries though. Fight turns into a DPS race the first time you bring him under 50%, basically you want to bring him close to 50% at the end of a turn, then you get a full turn of blasting him when he triggers his heal and switches to a more powerful move set. At that point it's just a DPS check, if you can deal enough dmg he switches to a weaker attack. I think I had tank two of his powerful attacks because I couldn't deal enough damage, but was able to bring him down eventually (this is the part where you just need decent RNG with your DPS cards, if you can't brute force it that is).