For this one, you need to lose to someone online, and then play them again in the next match and beat them. The easiest way to do this is if you are able to play at a harder difficulty like Hard or Expert, go down to medium. If you want some challenge to this achievement, go into a ranked match and purposely lose. After the match is over, go to their gamercard and make them a preferred player. Most likely, they will be going back to find another match, and you should too. If all goes as planned, you should be in the same match as them, and since it is on medium, you should be able to beat them. May take a couple of tries. This can also be done in a Player Match, so get a friend, or use the Achievement Trading Thread, to lose one song then win the next. You will go back to picking a song after the first round, and will be against the same person unless they quit out. You can both get the achievement in the span of 3 songs, you just need to lose the first song, win the second and lose the third so the other person can get the achievement too.