There are two types of bosses in the co-op missions, the heavy armoured minigun enemy, and the giant mech robot. The minigun boss is only available on Regular, Skilled & Hardcore missions, whilst the mech robot is only available on Skilled & Hardcore co-op missions. This achievement is easier done against the minigun enemy, simply work tactically with your teammate so you’re able to get in as many shots on his back as you can, as this is the only spot he will take damage from gunfire. The medic class is the best choice for this as you will have the shotgun and a defibrillator, both can do a substantial amount of damage against the minigun. It should also be noted that you will need to kill the boss after dealing half of the damage to it for the achievement to unlock.