This should come naturally, just walk up to a weak enemy and press X and you should get the Smash bonus. If for any reason you are having trouble with this at the end of the 2nd Chapter when you are in the sewage plant after saving your comrades, you wait for an elevator and Crazy monkey bots, the small brown ones will infinite spawn and will keep coming to you. Just walk up to them and press X, they are one hit, you should get this is no time as they clump together and you can get multi-kills. Make sure NOT to get on the elevator until you are done. Run around the elevator if you are hurt too much.