From Halopedia, the Halo wiki
Skull Combo is an achievement in Halo: Spartan Assault. It is unlocked by completing any mission with the maximum number of skulls active. The achievement is worth 10 Gamerscore on Xbox 360 and Windows systems, and 20 Gamerscore on Xbox One. It is represented by a circle with a skull.
An easy way to get the achievement is to utilize some combination of the Hollow, Famine and Black Eye skulls on a vehicle-heavy level, as the effects of these skulls are effectively negated once the player enters a vehicle.