13 0 0Pretty simple Achievement. Any level where you have your pistol look for any cluster of Grunts as one bullet to the dome will take them out. 3 in a row and achievement unlocked.
10 0 0This is simple enough normally but I thought I was worth noting that it also counts if you have the Grunt Birthday Party skull on and two other Grunts die from the small explosion created from headshotting the first. As long as they are close enough together they will all die and you will get the achievement.I did it on the very first level with the first set of Grunts on normal. Good luck!
4 0 0The spot I found easiest (on Legendary) was during the mission ONI Alpha Base. You'll get to a point where you defend an atrium, then have to ride up an elevator to progress further in the mission. While riding up, you'll get attacked by a swarm of buggers. These things only take a shot or two each, and there's probably 20 or more swarming. I got this achievement by accident.Happy hunting!
2 0 0I got this achievement today by loading up kikowani station right at the beginning. I was going for the fast and low achievements and when i turned left a group of grunts came and i just zoomed and shot them all in the head. Pop achievement unlocked.
1 0 0Here is a video guide by Halo Completionist who has done a guide for pretty much every achievement in the game.