Chief, Smash! Achievement in Halo: The Master Chief Collection

Once you reach the mid-way point of this final mission, you will find yourself inside a Forerunner armory. You’ll notice that there are plenty of powerful weapons to choose from here, but for this achievement, you’ll want to grab the Gravity Hammer.

After exiting the armory, you’ll soon come to a segment where Cortana’s AI chip must be inserted into a terminal. As Cortana hacks the Forerunner network, you will need to defend your position from countless waves of Crawlers. At this point, allow the Crawlers to swarm the central platform where the terminal is located. When they are all grouped together, you should have no problem in bringing the Gravity Hammer down and killing at least three of these pesky Crawlers with a single swing, thus unlocking the achievement. This may take a few tries depending on how spread out they are, but there are plenty of Crawlers during this firefight for multiple attempts at this achievement.