I've been trying for hours to do the super hit that significantly reduces his life and I haven't been able to do it. How do I do it or what should I look for?
To trigger a hit stop on the boss, you have to accumulate enough temporary damages (The temporary health bar on the boss' health bar UI), and then deal a power punch to him. That power punch consumes that temporary damage to deal even more damages. If you do it with a lot of temporary damages, you get a hitstop reward.