This achievement can be as easy as _“Insert Food Here”_. The achievement requires you to get 10 headshots in either a ranked matchmaking match or a campaign mission. Not total in all games but in a single game. I recommend trying for this achievement in the first mission Seirra 117 on the easiest difficulty. Once you get so far into the mission you will come across a destroyed falcon/pelican(air transport) and a couple crates near it. Go over to the crates and look around for a sniper rifle. There will be 2 of them there near the weapon crates. Then there will be a couple enemies for you to headshot. Just aim for the head of any enemy you wish and pull the trigger. Repeat this 9 more times(for a total of 10 times) and Bleep Bloop... you got the achievement. There are also Sniper Jackels along the way that you can kill for a Beam Rifle just do the same with the Beam Rifle and you can get the achievement. Hope that this guide helped you raise your gamerscore and get closer to 100%ing this game.
For the armored brutes scope in and then aim in the middle of their helmet towards the fleshy part of their face.