Costume Kingdom

This Quest will become available after you've completed the first Apex (see The journey begin's). Along with the other two "Bounty" trophies, it's highly recommended you pick this up at your earliest convenience. This particular Bounty is located in the Park Area (center of the map):

When it's active, you need to defeat (x6) Magmos' in battle (the Rhino looking Hallowmon with the flaming horn). It doesn't matter if it's a Goon fight, a Boss fight or a random world fight - any Magmos you defeat will count towards this Quest. Due to the RNG-nature of most of the fights, you want to pick this up early on so you have a better chance of completing the Quest prior to finishing the game. If you don't complete before finishing the game, you'll need to grind out the remainder post-game.

Once you defeated the 6 Magmos' required, return to the Bounty Board you initially picked up the Quest from. Interact with it to complete the Quest and the trophy will unlock.