Army of Two

You can actually get this in the training area. When you come to the first cover spot just keep fireing, stay in aggro for 1 min and your done.

Watch the video:

In this video I will show you how to get the "Fear Is The Mind Killer" achievement and how to complete the training mission as well.

Once you have completed the game once start up any mission and equip yourself with the minigun (preferably pimped) and then put you AI partner on standby (blue) and tell him to hold position. Then just put yourself in cover with about 4 enemies shooting at you and blindfire at them. Then wait till you have full aggro as shown by when your side of the bar goes red and do this for 1 minute. If you run out of ammo on the minigun switch to your sidearm, a sub machine gun is a good idea for this.

If you want this achievement and obtain it via a single play thru, just hope on a turret and keep shooting at a group of enemies; Note dont actually shoot the guys just shoot the cover they are using and before you know it, you will have it.

The way i did this was sign in another person and go to the Iraq mission. Go up to the first box where the turret starts shooting at you and hide behind it with the profile that you want the achievement on. Then use the other controller to catch up to your other guy and have him run to the right behind the tents and let him get shot enough to where he can fein his death. This will cause you to get full aggro and it wont go away or even lose a little until the other person gets up. Behind this box you should be safe from any crazy iraquees trying to rush you. Just last a minute or long enough until the achievement pops and there you have it. Good Luck!!