Hi everyone, I'm fairly new to this game, and when I watch speedruns, usually the runner kills Solgryn during his second phase before the green shield, but I can't seem to damage the boss during this time, can someone tell me what am I doing wrong? Tried with and without double shot glitch, his hp at the top of the screen doesn't move an inch

His HP bar goes past the screen, so you have to shoot him more than what's on the screen. Try shooting more and you'll see it go down.