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Event Achievements almost always require you to more than just the main storyline in order to collect enough items to complete them. Some require doing Repeatable mines & others require at least some of the Challenge locations be done (as in this case.)
The Achievement requires you to collect a total of 44 Capes. There are only 34 Capes in the Main Story. So you must get the remaining 10 Capes from either doing Challenge 1 (has 11 Capes) or Challenge 2 (has 12 Capes.)
Here's the Event info. You can pick your region & the chart tells you how much energy is required for the Challenges so you can decide which one to do. Though you have plenty of time to do both 🙂
https://wiki.diggysadventure.com/index.php?title=Superheroes_2
