The Enemy of my Friend - 100G
Get a higher individual score than your partner on all co-op missions.
This achievement is pretty easy as the co-op campaign only has 5 missions that are pretty straight forward. You need to beat your co-op partner in all of the 5 mission, this doesn't have to be the same partner you can just play all of the missions with different people if you wish.
It can be easily done with two people, with both of you getting the achievement. I personally played with my friend normally through the co-op missions managing to beat him on every mission. This gave me the achievement, after this we played the 5 missions again with him getting higher scores than I did. This gave him the achievement.
I believe that if one of you uses the score multiplier it counts for both of you, but I'm not 100% sure about that.