The earliest you can get this achievement is at level 7 with the xenos, using the lurker or spitter classes. You will need to purchase one upgrade in each slot (starter skills will not do, you must spend an attribute point gained by leveling up or doing challenges, on the skill or mutation) for only one of said classes in the slots the game provides you for loadout options, AND have them equipped, simply buying them all will not pop the achievement. If you are having trouble getting xp, try and knock some challenges out while playing as they're worth 500xp a pop. You can also do them in 1v1 private matches as well as some of them such as the spitter kills challenge can be somewhat tougher depending on your play style.