Nickelodeon All-Star Brawl 2

In the Campaign, between runs in the hub, there is a shop on the main level, under the portal. Here, you can buy perks for Green Slimes, and upgrade purchased ones with the same. For this achievement, you need to upgrade any perk to its max level. For most perks, this is level 3, but some go up to level 5. Obviously, for the purposes of this achievement, choosing one that goes to level 3 is ideal and cheaper. Personally, the two perks I recommend the most are the one that starts you with extra stocks (this is practically mandatory), and the one that lets you heal when inflicting debuffs (hugely helpful when paired with the powerup that lets charge attacks inflict debuffs). This latter perk only has three levels and is easy enough to max out after one or two runs. The former has five levels and is quite expensive, but if you're focusing on only those two perks, you'll likely max out both by the time you finish the story in the campaign.