Get 30 headshots in the South China Sea mission in the Campaign (20G)

For this achievement, you'll need to get 30 headshot kills during Mission 3: South China Sea. There are easily 50 enemies on this level, so you won't need to get that many headshots. Also, stay away from RB as this will make your teammates engage, and kill enemies.

I recommend using the following weapons, which you should have unlocked by this mission:
- MK11 Mod 0 (Short/Medium Range)
- Scout Elite (Long Range)

If you don't have these weapons, make sure you use a power single shot weapon. Just make your way through the level, and get as many headshots as you can. Use cover, and take your time making sure to get the kill. You'll get a headshot confirmation in the points that show up at the bottom of the screen.

If trying to line up really long distance shots, make sure you use the D-pad to compensate from bullet drop.

Once you get 30 headshots, the achievement will unlock.