They weren't quite back to back, but even without seeing Trunks could sense her ki, and her frustration. Besides, smashing rock wasn't exactly subtle. Turning his head just enough to keep an eye on her and on the rock-golem's reforming arm.

Soon though, there's a flash of something shiny that isn't green-energy. "One good one, Cass!" Trunks says, letting off a quick blast at the reforming golem-arm but most of his attention on where he'd spotted that shimmer.

There! He turned fully, one arm moving around Cassie's waist to pull her close as the other hand dived in to grab the crystal. Quickly. Before using that same speed to move them a few feet away as the golem just...crumpled.