Given the structure you've outlined for "Power Rangers Cosmic Fury Season 2," it would make sense to have Grum and Across defeated in Episode 10: "Once and Always A Ranger." This approach allows for a more climactic and emotional finale, where the Rangers can use everything they've learned throughout the season to confront and ultimately defeat these villains.
Episode 5: "Battle for the Grid" could serve as a pivotal point in the season where the Rangers face significant challenges and setbacks but do not finish off Grum and Across just yet. This would build tension and anticipation for the final showdown in Episode 10, allowing for character development and plot threads to culminate in a satisfying conclusion.
This structure would give the season a well-paced narrative arc, keeping the stakes high and maintaining viewer engagement throughout.