The Gorosei with their insane haki and abilities, Garling and now Shamrock, the HKs
Ryokugyu who has a confirmed black blade who almost packed up the entire Alliance if he wasn't jumped, Kizaru who faked defeat and fed the protagonist during their duel, 5 Billion Sakazuki, and supreme grade Fuji, Garp/Sengoku....
Back muscles should be the hardest to tear, what type of lifting were you doing?
(are you roided?)