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.