From Halopedia, the Halo wiki
Beachhead is an achievement in Halo: Combat Evolved Anniversary and Halo: The Master Chief Collection. It is awarded for not losing any Marine allies during the beach landing on The Silent Cartographer, on Heroic difficulty or higher. In the original game, it is represented by a white MJOLNIR Mark V helmet over the outline of a Anskum-pattern plasma grenade. In The Master Chief Collection it is represented by a screenshot of Boarding Action in the Xbox art, and is represented on Steam as an enclosed Marine CH252 helmet in front of two crossed difficulty swords - signifying the difficulty required to complete the achievement.
The achievement is worth 10 Gamerscore.
The player needs to take the damage for the Marines. Keep moving, and make sure the Marines are behind you. Throw grenades when the Covenant troops are in cover, and the use the pistol when they are far away. Don't let the Marines get close to the Sangheili or the Plasma grenades' blast radius.