Edit 2/1/13: Apparently, your xbox live friends to do not count for this, only frenemies that you add by manually clicking once on their name on the leaderboard at the end of a round.
(credit to Gacys Clown and JGoDzzz1 for verifying this)
What I did was play a round and only get a couple words correct so that you are on the bottom of the leaderboard. When the results show, you can see people that are near you on the bottom of leaderboard. Click on one of them to add as frenemy (there will be no indication that they were added).
Then just hope they play the next round and do poorly again, but this may be hit or miss. Often, the people near the bottom are just grinding out rounds and not really trying, so they should be easy to beat.
It may also help to pick someone from another country that is not playing their native language.
On the main menu, you can click on the frenemies button and follow and unfollow people this way too.
Just to clarify, this is not the leaderboard that shows all your xbox live friends at the right of the screen. This is just a high score leaderboard out of all the games that have been played by your friends. The leaderboard that matters, is the one that appears at the end of a round and only includes frenemies that played that particular round. If you have a frenemy online playing, yours and their score will be shown on the top of this leaderboard.