From Halopedia, the Halo wiki

HSA - Demolition.png

The Demolition medal is awarded in the Halo: Spartan Assault and Halo: Spartan Strike campaigns for destroying a structure with a grenade.

The medal is most easily achieved in the Windows version of the game, utilizing a mouse, as this control scheme gives the player far better grenade control than is offered by a controller or touch controls.