The only thing is, in a 1 on 1 scenario, the dearth star could shoot the cube and it wouldn't have time to analyse or send back data to be analysed for the next Borg cube to become immune to the Death Star shot. If there are 2 cubes, we can also assume there can be 2 death stars, and coordinated shots would achieve the same result. But if the Borg cubes are more than the death stars, or if the shots have a bit of a delay between each Death Star, the data could be analysed and transmitted back for other Borg. If a sith was involved, he could use the force and I don't think the Borg can find a way to become immune to the force. And then the sith can destroy any number of cubes or drones or mother borgs or whatever. A sith might not do that though, which might be their downfall, as they might want to control the Borg and create a great army instead which could lead to enough possibilities for the Borg to win.