Costume Kingdom

The Cranky Crocross will be available after you've completed the first Apex (see The journey begin's). This is not part of a Quest, it's just a "world boss" that you can defeat. He’s located in the Lake area on the left side of the town:

If you don't already have it, make sure you pick up the "Bounty - Crocross" quest from the billboard at the entrance to the lake. You'll be able to complete both quests in this area. Make your way across all the piers/platforms/ledges and you'll eventually reach the top of a small mountain where Cranky Crocross is waiting for you. Defeat him in battle and when you return to the open world, go behind where he was sitting to find a chest. Interact with the chest to pick up the Crocross Costume/Hallowmon. The trophy will unlock when you do.