I'm two achievements away form S ranking this game. All I have to left are two co-op achievements: complete all the missions in co-op and "complete all instances of activities in co-op." I really hope the second one means "complete each of the activities in co-op once" and not "complete every activity in coop."
I beat the mission where you can choose blow up the Morning Star skyscraper yesterday with a friend (who isn't after this achievement). It would be great to have more people co-op with. And if you're missing these or any other achievements, I'll be glad to help you get them.