World Beater.

To get a gold grade on a world you need to have a gold grade on all 3 of the levels in that world. In order to get a gold grade on a level make sure you collect & smash everything, find all 3 collectables, open all chests and get the stars, kill all enemies, and win the HEX challenges.

If you loose a HEX challenge just quit out to the main menu and go back to the world and try again (you will continue on the sub-level you quit on).