The Amaranto and the Promethea were both expert rifleman mechs, but fought in completely different ways.
The former was the classic marksman mech that sat back far in the rear and took down high-priority opponents by targeting their weak points.
Normally, that worked out great because every mech or starship possessed a weak point. Whether that was a hole in the outer surface, a joint with much less armor cover or the rear of a starship where the thrusters were dangerously exposed, there was often a way for the Amaranto to turn a battle around by crippling a powerful opponent with one, dazzling precision strike!
The expert rifleman mech was not omnipotent, though. If that was the case, then the Golden Skull Alliance would have defeated its previous opponents with much greater ease.
The greatest hindrances that impeded the Amaranto were energy-based defenses and lots of junk in the way.