From Halopedia, the Halo wiki
Foot Soldier is an achievement in Halo: Spartan Assault. It is unlocked by completing the objectives in Operation E, Mission 4 without using any vehicles or turrets. The achievement is worth 10 Gamerpoints on Xbox 360 and Windows systems, and 20 Gamerpoints on Xbox One. It is represented by a circle with two footprints inside it.