In JP, I think they used Denzel's Taunt "Dead Man's Revenge" to do the damage.

Oh true

But this bosses 2nd phase is hard

Well he's fairly easy if you have blue valenty and a high dps unit like blue escanor. A unit like escanor can just one shot or near one shot ban in the second phase using ult. Valenty is good because of ult gauge lock, detonate, and rupture ultimate for second phase. Blue Lilia is good because of ult gauge removal and heals

I saw your units on the other post you commented on and you're really new to this game, final boss shouldn't be your priority at all right now. Your units are not ideal for this boss

I dont plan on doing him anytime sooon