This week 10 epic quest requires you to destroy special sandcastles.

This is possible at 2 locations. The first is east of Craggy Cliffs. There is an area on the beach where destroying 3 sandcastles is possible. The second location is south of Flushed Factory on the south side of the map.

You will see sandcastles and when you approach it will alert you that you can destroy it. Just destroy all 3 in whatever location you go to and the quest will unlock.

Maps with both locations are shown below.

Map credit to fortnite.gg