WoWDB

To complete this achievement you must collect all of the following Challenge Cards and defeat all of the following brawls:

• Rank 4: Grandpa Grumplefloot -  Knockoff Grumplefloot: Unlocks the Grandpa Grumplefloot brawl - obtained by doing a /dance emote on Grandpa Grumplefloot.
• Rank 4: Master Boom Boom -  Hozen-Fur Fuse: Obtained randomly when killing Hozen while doing the The Burlap Grind daily quest.
• Rank 4: Ty'thar -  Raptorhide Boxing Gloves: randomly obtained from Kor'kron mobs in Northern Barrens.
• Rank 5: Dippy & Doopy -  Frost-Tipped Eggshell: Obtained after defeating  Doopy, a level 25 Legendary Battle Pet in Icecrown.
• Rank 5: Razorgrin -  Impeccably Sharp Tooth: Obtained from fishing in Pandaria.
• Rank 5: Splat -  Vial of Reddish Ooze: obtained from the  Greater Cache of Treasures rewarded for random heroics and random scenarios.
• Rank 6: Blingtron 3000 -  Dusty Old Robot: Obtained from the  Blingtron 4000 Gift Package.
• Rank 6: Mecha-Bruce -  Modified Chomping Apparatus: Obtained by defeating Bruce after fighting him with  Challenge Card: Bruce.
• Rank 7: Argh & Raaaargh -  The Bear and the Lady Fair: Obtained from the  Greater Cache of Treasures rewarded for random heroics and random scenarios.
• Rank 7: Ro-Shambo -  Paper-Covered Rock: Unlocks the Ro-Shambo brawl - obtained from harvesting crops at your farm in Valley of the Four Winds.
• Rank 9: Blind Hero -  Well-Worn Blindfold: random reward from  Darkmoon Game Prize.
• Rank 9: Mingus Diggs -  Digmaster's Earthblade: Obtained from Tol'vir Archaeology nodes or from  Crate of Tol'vir Archaeology Fragments.

This achievement will become a Legacy achievement in Warlords of Draenor.  I imagine a lot of players wish it wasn't, though - if they're like me, they've been holding off on it because they'd need to somehow make room for storing the shirts.

Upon completing this achievement, the character will receive a letter in the mail from Boss Bazzelflange.

Let's take a quick inventory: You've defeated a shark, a robotic crocolisk, a pile of ooze, a pair of penguins, and a whole roomful of deadly brawlers from around the world.

Yeah. I'd say you earned a shirt.