Brawl Star

Step 1: Earn Rank 8 with the guild
Step 2: Purchase Challenge Card: Dippy
Step 3: Choose one of the 8 affixes from [H] Challenge Broker Guo / [A] Challenge Broker Senna
Step 4: Go down the list of affixes as you continue to fight Dippy each time

Enjoy your achievement

For the Horde - Speak with Challenge Broker Guo.
He is located at 51.26, 68.56 in the Brawl'gar Arena.

For the Alliance - Speak with Challenge Broker Senna.
She is located in the Bizmo's Brawlpub, near the arena (sorry, I play Horde!).

They will have a list of options, each relating to a different achievement:
Arm the audience with foul fruit! = Audience Participation - During this challenge, the audience throws food into the arena, just dodge this and do the fight as normal.
Please freeze the arena! = Brawler's Chilled - During this challenge, the arena is filled with ice and you will slide over it as you run around, do the fight as normal while navigating the ice.
Make me a giant! = Brawlzilla - During this challenge, you have the Maximal Magnitude buff, greatly increasing your size, do the fight as normal while a giant.
Literally set me on fire! = Hot Footed - During this challenge, you have the Hot Footed buff. While you have this buff, you cannot remain still and leave a trail of fire. Do the fight while a running around on fire.
I can win without a scratch = Let me Solo It - During this challenge, you have the Frangible Field buff. If you are hit by the boss, this shield will be removed and you will not earn the achievement. Defeat the fight without taking any damage to earn this achievement.
Fill the arena with sticky molten tar! = A Prime Ordeal - During this challenge, the room is filled with tar, slowing you, and sometimes fire will shoot from the ground, just dodge this and do the fight as normal.
Make me as light as a feather! = Featherweight Brawler - During this challenge, you have the Lightweight buff. I could not see any discernible differences to my gameplay with this buff. Do the fight as normal I guess? After doing a few fights with this, I think it slows your fall speed if a mechanic knocks you into the air.
I can win with just my bare hands! = Bare Knuckle Brawl - During this challenge, you have the Bare Knuckle Brawling buff. This one removes your equipped weapons which can be challenging for abilities that require a weapon to be equipped. Do your best to complete the fight without a weapon by utilising spells or melee swings.

If you complete all these challenges, you will earn Brawl Star which earns you the title "Brawl Star <Name>".

I found all these achievements easiest to do at rank 8. Once done, I purchased the Challenge Card: Dippy from Card Trader Ami (The Alliance vendor is Card Trader Leila).
This is a simple fight that allowed me to zerg Dippy while dealing with the extra fight component.

Enjoy your achievement and your new title after collecting them all!

Soo... you can be in a group with someone who's AFK in the Brawler's Guild, and they get credit for bosses you kill. Just found out when my friend got this achieve from my kills.