Load up at Oni Alpha Base at Rally Point Charlie. Make sure to grab a shotgun at your earliest convenience. Once you get to the top of the elevator, look for the three Brutes with jetpacks. There are only 3 of them, so make sure you get them all with the shotgun. Once the 3rd dies, you'll get Fowl Hunter.
I've included a video below with loads of achievements. For the Fowl Hunter achievement, start at 8:18