Army of Two (EU)

do this in the tutorial.
when you come to the turret, begin shooting anywhere (you have unlimited ammo) as your partner does nothing to get the full aggro on you. keep on doing so for a minute.

You can this this achievement during Somalia (tutorial level), progress through the level until you reach the turret, you can shoot anywhere and it will build up your aggro bar and after about a minute you'll have the achievement.

The reason it's recommended to do this here and get it out the way is because you'll have infinite ammo so you can just hold your trigger down until you get the achievement, also, if playing single player your AI partner won't do anything to subtract from your aggro!