Jack had been looking for his opening to help, and the Left Side of the thing holding his friend in place is it. Peaceful solution, sure, until the instant that you're endangering a friend, and that thing will kill Chiaki if left alone. Mirage Dive sends two clones of himself with copies of his spear to attack at the Left Arm of the WEAPON, while Jack stays back, himself. With the number of area of effect attacks being slung around, he doesn't trust himself to not fall to friendly fire.