The guides already up for this achievements are great. But if you don't want to play cheap, and you're not quite MLG material, there is another method for this one- look for the easy fights. I found that hosting matches tends to hook you up with the lower level players a lot more often than searching for matches. But another way to fight the easy fights is to look at the gamertags of those you are playing. It can be easier than you think to pick out the 13 and younger crowd just by examining their gamertags.EliteKid97? Pushover. Halofan43? Never blocks. While it may take some searching to find a bonafide easy fight, it is worth it for the 50 gamerscore.
If you don't mind playing cheap though, but don't want to lose your rep, you can always quit out at the "VS" screen when the players' rank shows up. If you're outclassed, this is the place to see it. No one ever gave me a bad rep for quitting at this screen on my path to this achievement.