Easiest way to unlock this is simply to get 1 other person and fire up the campaign mode, set the difficulty to singular and just play the very first mission which is the tutorial. Achievement will pop after u finish the mission. I actually started the mission solo and a friend joined me before finishing the mission and we both got the achievement at the end of the mission.
2 players at least
Play mission 1-1 (Tutorial)
Achievement pops after you defeat the armored enemy and finish the mission