Get on one of the gun turrets you encounter, and keep firing. This will fill the Aggro meter and keep it full for the one minute requirement to get the achievement. This is easier to do during the second mission.

You can get this achievement with the M134. As soon as you have it, go to the Afghanistan level. After you pass the first wave of enemies, you must go through a tunnel. If you are playing alone, keep your AI behind where he cannot shoot enemies. Run pass the first set of sand bags, and get to the second set in front of you. Use the M134 to shoot any incoming enemies. Then, just keep shooting until you get the achievement. If you are playing in Co-op mode, do the same thing; one player stays behind and the other one shoots. Restart to the last checkpoint if the other player also needs the achievement.

Play the first level. While going through Aggro Training you will have to use your partner to distract an MMG turret that is being fired by Pedro. Shoot at the turret for a full minute to raise your Aggro. You will have infinite ammunition, so you do not have to worry about running out.