I'm Your Number One Fan (Season 2)

I'm Your Number One Fan (Season 2) is an achievement earned for cheering a player on after they have won a brawl before the  release of Patch 7.0.3.

Strategy
Enter the Brawl'gar Arena in the NE part of the Valley of Honor in Orgrimmar (or the Deeprun Tram in Stormwind City for Alliance). Once inside, simply wait for a player to win a fight and  him/her to earn the achievement.

Be sure to target the player when you  and if you are finding that you aren't getting the achievement make a macro as others have suggested.