The Goose is Loose Achievement in Halo: The Master Chief Collection

In this gametype, two teams will face off in a “cat and mouse” sort of chase. On one team, there is the Juggernaut who acts alone in an attempt to kill as many players on the opposing team. Each kill the Juggernaut obtains will is worth one point. On the other hand, the other team consists of the remaining players in the match and their goal here is to kill the current Juggernaut. After a player kills the Juggernaut, they themselves will assume the role of Juggernaut and can begin amassing points by killing players on the opposite team.

This can be obtained in a Custom Game by yourself, so just load up the aforementioned game type and when the match begins, you will automatically spawn as the Juggernaut. All you have to do is wait thirty seconds and the achievement will unlock with ease.