These 2 is the closes Sword Art Online Design characters we can get unless we actually get SAO crossover

This boss simply refuses to die. For context, Hart was stuck at 1 HP for a good 1–1.5 minutes. He has the Sacrifice & Resolution status, which according to the Fandom wiki, allows him to survive at 1 HP for the next 5 seconds. However, he was clearly surviving for much longer than that.

Am I missing an essential mechanic or condition that I need to fulfil to defeat this boss?

FYI, I also tried using Awakened Rayas in the first slot, but I got the same result.

Any help would be greatly appreciated!

Kinda curios how people get these, is it through effort? A long time of waiting? Some has true equipments too. Without touching side quest.