Leave It Where It Lay
From Halopedia, the Halo wiki
Leave It Where It Lay is an achievement in Halo: Combat Evolved Anniversary and Halo: The Master Chief Collection. It is awarded for completing the level Two Betrayals without picking up any other weapons on Legendary difficulty. It is represented by an MA5B assault rifle with a "forbidden" sign over it in the original game; in The Master Chief Collection it is represented by a screenshot of Death Island.
In the original game it is worth 25 Gamerscore; in The Master Chief Collection it is only worth 10 points.
One easy way to earn this achievement is to start the mission on Legendary with the Bandanna skull active. As the skull grants infinite ammo, one never needs to grab another weapon, so the achievement simply becomes a matter of remembering not to pick anything up.
Another option is for the player to play cooperatively on another profile while leaving their own controller untouched. If the player has not already earned them, this method can also potentially unlock the Look Out for the Little Guys, Standard Operating Brocedure, Brovershield and Bro Hammer achievements.