Here is the full list of all 18 LEGO Brawls achievements.
Master the Cowgirl Champion
Be named Top Brawler 20 times
Master the Janitor Champion
Master the Mummy Queen Champion
Master the Punk Pirate Champion
Master the Owen Grady Champion
Create 3 custom brawlers
Master the NRG Nya Champion
Play 20 Brawls
Win 5 consecutive Brawl matches
Be named Top Brawler 5 times
Play 5 games in Party Mode
Play 5 Brawls
Be named Top Brawler
Play your first game in Party Mode
Play your first Brawl
Customize your Loadout
Customize your first Brawler