I wouldn't know about the challenges, the only weapon I got 150 head shots with was the MP40 (my favorite gun too). Also, I wouldn't rely on teammates for anything, unless you actually know the person. There's been times where a teammate was facing me and I still manage to get knifed without my teammate taking a single shot. Here's my most used class setup if anyone's curious: Gun: MP40 (silencer) / Magnum Grenades: Tank / Gas Perks: Bomb squad, Stopping power, Steady aim A strategy in case you guys don't know, wait to use dogs until you die. It'll get you started on another kill streak.