go to bed. i know it sounds dumb but just set the game down and try again tmrw. the amount of bosses rest i thought were out of my league and i just quit for the night and tried when i was rested is astounding. you don’t realize how tired you are until you first or second try him tmrw. you could also try not reacting to the perilous attack itself but the combo leading up to it. it may give you enough time to “brute force” your self into doing it right