Each Starfighter gets Critical Hit 3 as long as it's attacking the Death Star.
Each player discards all of his or her units in the Ground arena. Play only if your opponent has no units in the Space arena and only when the Death Star would attack.
You can't use this ability if your opponent has any units in the Space arena, even units that don't control the arena.
Space units in your opponent's build zone have no effect on whether or not you can use the Death Star's ability.
When this ability is activated, all your units and all your opponent's units in the Ground arena (including their Pilots, if any) are discarded. Units in your build zone and your opponent's build zone aren't affected.