From Halopedia, the Halo wiki
Overachiever is an achievement in Halo: Spartan Assault, exclusive only to the Xbox 360 and Xbox One versions. It was released along six other new achievements in December 2013. The achievement is awarded for earning all co-op medals at least once. It is worth 50 Gamerscore on the Xbox 360 and 100 Gamerscore on the Xbox One. It is represented by a circle with a medal inside it.
Unlike the Distinguished Service achievement for which campaign medal totals carry across platforms, co-op medals in the Xbox versions of Spartan Assault do not. As such, players must earn the co-op medals at least once on each platform.
List of Medals