He's going for the Endure Achievement, not the Classic Achievement.
Do it on ONI Alpha Site definitely the easiest because all the covies filter into on area from only two entrances. Keep two people looking down each entranceway and try to take them out as soon as possible. When Fuel rod grunts come in make sure you take them out first. And also hammer brutes, or brutes in general, use a charged plasma pistol to take out their shield/armor and then one shot head shot. I noticed through my play through that I was running low on ammo when the famine skull turned on so headshots had to count. When hunters come into the picture you can either go into the elevator where they cannot go into or have one person pull them and the others shoot them from behind....Should take only one run if your good enough, but it might just take some more. But Alpha Site is by far the easiest to do it on