This achievement is very easy and doesn't take long to get. All you need to do is play a few rounds of Combat Training, which is basically CoD's multiplayer mode with bots. Put them to Rookie difficulty if you suck at multiplayer shooters or if you want to laugh at REALLY stupid bots. It took me around 4 Team Deathmatch rounds to get to level 10, so you're looking at roughly 40 minutes of playtime if you are a mediocre player. Good shooter players can get this much faster.