Of course the Black Lord was armed with missiles.
The ace mech was not armed with generic missiles either.
The ones that exploded against the First Sword Mark III happened to produce more forceful explosions, making them good at pushing enemies away.
The first-class ace multipurpose mech also appeared to possess a sizable reserve of missiles. It continued to fire a salvo of missiles at clever intervals, thereby repeatedly interrupting the First Sword's momentum and causing her attempt to fail.
Since the First Sword Mark III lacked any form of point defense, the missiles quickly converged on the Larkinson ace mech and exploded!
The concussive forces unleashed by these simultaneous blasts were already powerful to shatter regular first-class multipurpose mechs. Even expert mechs could not avoid damage due to the resonance empowerment.