Fowl Hunt Achievement in Halo: The Master Chief Collection

This achievement can be obtained towards the end of Oni Alpha Site and is best done when playing on a lower difficulty (thus making the Brutes easier to kill). Once you reach the interior of the ONI Building, one of your Marine allies will almost always wield a Shotgun. At this point, swap one of your weapons out for their Shotgun and hold onto it (and don’t expend too much ammo) for the duration of the interior firefights. 

Once the initial waves of Covenant are dealt with and you’re required to take the elevator up to the roof of the building, you’ll be greeted by a Brute Chieftain and about 4-5 Jetpack Brutes. Make haste and dispose of at least three of these Brutes with your Shotgun (usually 2-3 blasts will kill them) and the achievement will be yours.