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It could be a c4 (planted on a wall) or a mine (planted on the ground). Simply wait for an enemy patrol to pass over/near one of these and then snipe the mine/C4, killing the enemy nearby it. There are so many opportunities to get this, that you shouldn't even have to try really. You could get this at any point during the campaign, or during the dynamic missions, during HVTs, or even during your co op playthrough.