To beat the Challenge called "How in the ?" you have to use bullet penetration to blow up an explosive device thru a wall and kill an enemy player. It was easier then I thought it would be. On Vacant I used a deep impact perked M16 to shoot a claymore I saw around a doorway. I'd suggest setting up a P90 with double tap and deep impact. Start spraying around the doorways of this map. It's probably your best chance.
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