Costume Kingdom

This quest will become available once you complete the first Apex (see The journey begin's) but can't be fully completed until you complete the second Apex (see The push). On the left side of the park you can find Rex:

Interact with them and they'll give you a quest to find 6 missing Hallowmon. You can get 4 before you complete Apex 2 but you won't be able to access the areas for the last 2 until you complete the second Apex. Their locations are as follows (marked with Pokeballs, because why not):

To "find" the Hallowmon, you need to defeat them in battle. Luckily, you're able to see the Hallowmon before you start each fight so rearrange your starting line-up as needed to ensure the Hallowmon you're using can easily win the battle(s). Once you've "found" all 6, return to Rex. Interact with them to turn the Quest in and the trophy will unlock.