Yeah I had the same issue, I kept quitting lone wolves to try and get the ascetic helmet so I went from a Lutenint Grade 2 to a Gunnery Seargent. Eventually I got the achievemnt a well as the Overkill achievement at the same time and ended up liking Comanndo better... Any way, once I got back to lutenint I was wondering why I dind get the achievement so I kept playing and now am a Captin. So thanks and now I know how to get it