Battle of Sunaion (EasyDifficulty:
Near the beginning of the mission after the first door opens and you see a turret in front of you, keep left and enter the first building. You'll notice a half-circle staircase with a golden circle on the ground nearby with some grunts. Walk at least halfway up the stairs, and ground pound (jump and hold melee) into the grunts and grate. As long as both are in the blast radius, the achievement will unlock. If you fail, reload the checkpoint.