Deck Your Collection (Season 2)

It is quite easy to do in 7.3. You buy [item=143763] and give it to Bizmo's Brawler Bouncer, and then you keep fighting a random enemy.

I noticed once you defeat enemies from this achievement you will not fight them again, so that makes a bit less random. I was able to finish it in 3 hours.

I can confirm as of 7.3.5 the achievement is still able to be gotten, and the shirt can be recieved.

Here is proof of the achievement popping up: -

https://i.gyazo.com/28888d56dc96c0b02ff1f7f5741c8826.png

and here is me receiving the shirt from the mailbox: -

https://i.gyazo.com/ecf74ef106b409e60b04bc70f801db4f.png

This achievement was added to Legacy. Due to such, there is no way to find the achievement unless you've already earned it. That also means you can't track it in the tradition way.

As some have pointed out on the Shirt's page's comments, you can still obtain this achievement in 7.3.

Here is a script for tracking it:
/script AddTrackedAchievement("9177")

I was one off and couldn't figure out which one, so I decided to use the script. I then realized which one it was, did random brawls until he popped up, then got my achievement upon killing him.