When you get to the Nuclear Power Plant stage, Smithers is the first boss you'll face at the level's end. One of his main attacks is to throw bombs at you. A few of the bombs he throws will remain on the ground, and you can pick them up and use them like regular items in the game. Simply pick one up and throw it at him and you get an easy achievement. The number of bombs he drops seems to differ depending on how many players you have. On a 4-player game I was in, he didn't drop a single bomb that could be picked up.