The second both sets of manacles are open Fives throws himself at the creature, a lifetime's worth, however short that life has been, spent training to fight and kill brought to bear with all the skill at his disposal on trying to find pressure points, weak spots, something. Anything. Literally fighting tooth and nail, because there is no such thing as fighting dirty, just fighting for your life, and he promised he wouldn't hurt himself, he didn't say anything about the monster he's been sacrificed to.