To complete the Ewok and Roll Challenge in Lego Star Wars: The Skywalker Saga, you need to blast the loose rocks with the AT-ST to take out Stormtroopers during The Chewbacca Defence level in Episode VI – Return of the Jedi. You’ll complete this Challenge towards the end of the level, after completing the Sweep the Legs Challenge, collecting at least some of the Minikits, and commandeering your very own AT-ST.
The loose rocks you need to blast are on the right hand side of the AT-ST battle section of the level, between the front two AT-STs and the rear two. The most important thing is not to defeat all the Stormtroopers on the ground before you blast the rocks. Your AT-ST will lock onto Stormtroopers on the ground when you aim, so don’t get too trigger happy, and aim away from the Stormtroopers towards the bug pile of rocks with an Ewok jumping up and down on top of it. Don’t worry about the Ewok — those furry critters are very good at getting out of the way at the last second. Shoot the rocks with just one blast of AT-ST fire and the whole pile will collapse, smashing into the Stormtroopers taking cover among the boxes and crates in front of the rock pile, and this will complete the Ewok and Roll Challenge.