From Halopedia, the Halo wiki
Dynamic Duo 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 Gold stars in all the co-op missions. It is worth 30 Gamerscore on the Xbox 360 and 60 Gamerscore on the Xbox One. It is represented by a circle with two Spartan helmets inside it.
The Dynamic Duo is a common nickname for the team of Batman and Robin.