The Fighting Robot, a combat mech designed specifically for close combat, is equipped with a 'Combat Circuit.' In an unobstructed one-on-one close-quarters brawl, it's an almost invincible entity.
The reason why Mu You's mech is performing so brilliantly now is due to its Chaos Circuit. When the Chaos Circuit is equipped with fast attack firearms, it can plunge an entire army into chaos, thus making it impossible for them to perform at their full strength.
However, the Chaos Circuit can only affect long-range attacks, proving ineffective for close combat. Hence, Barney made a decisive move by deploying the Fighting Robots, a type of mech that isn't commonly used.
[You see all the long-distance robots cease fire and retreat, giving way on the battlefield. Nearly a hundred Fighting Robots charge at you from all directions.]
[You hastily spray rounds, but the low-damage energy bullets prove ineffective against the Fighting Robots. They ignore your attack and continue to advance.]