You're probably just missing the last boss stuff.
Just watch that video and you only charge the first attack (hold down triangle).
His opening is to dodge the opening attack, then Y attack with the pincers to eat some damage. Maybe I should do that since I attempt to charge Y with the blades, then evade after I release the attack to have i-frames in the dodge of the attack but either way I'm not doing as much damage as he is from what I'm noticing. :/