Nickelodeon All-Star Brawl 2

This just requires you to play as all 25 standard brawlers throughout the 100 battles you'll be doing for Defender of the Earth (100G). So, while grinding out those battles, just make sure to switch your brawler after each one to pick the next brawler, going down the list (because there's no way to track which you've played as) until this achievement unlocks. As noted in that achievement solution, the quickest way to run through battles is to play with a single stock, and as soon as the match starts, kill yourself to end it.