To get this achievement you have to purchase a costume from the guardian golem shop. Any costume piece will unlock this. In order to find the shop head to the right to where the robot guardian is. Double jump on the red bounce pad to the left of him and continue left into the treehouse. On the left, there will be a fox-switch jump near it and press
to activate it. Doing so will lower a drawbridge. Cross the bridge then head straight back, to the shop, you can't miss it.