In Rustwood Grove there is a... colourful... character, that tasks you with chasing off the birds that keep "painting" his house.
Instead of putting the mask on, just stand outside the house. Lili will keep prompting you to put the mask on, but don't be tempted until the achievement pops.
The alternative--running around chasing birds--is a phenomenal waste of time. If you've already finished the quest, don't worry too much. It's way quicker to start a new game in a different slot and play up to this point than it is to run from place to place waiting for birds to respawn.