This boss is located at the end of the 'Pacific' act in the mission 'Farnsworth'.
Unlike the other bosses, it is damaged through attacking weaknesses around it's body. You will only damage the boss when the weakness has turned orange. Be sure to place two or three emplacements down around yourself to watch your back as you attack the boss. Like all bosses it has three health bars, once a health bar level is destroyed it will retreat for a short amount of time and send variants of Monovisions to attack you.
If you have turrets placed be sure to watch out for roving groups of Knobs as they will turn them into dust. Once the first weakness on it's head is destroyed, it will raise itself up slightly revealing three more weaknesses. When these have been destroyed it will yet again raise itself for the last time to reveal a further four weaknesses.
This achievement requires you to kill it with more than one player in Co-op.