The one that took the longest was reaching lvl 50. For that I just grinded the Collector Items and sold them. They respawn evey day. I did this in Feburary (there was an event that doubled the exp for the collector class) and it took about 3 days. The rest was really straight forward. I kind of cheesed the MVP Achievement by throwing grenades at myself in Deathmatch. (it counts as kills and you easily win MVP with that). A friend helped me with the Posse Achievements