There is context for that though. Kingdom Hearts was corrupted by darkness because Xehanort opened it. The same thing happened during the final boss fight. While Sora was in Scala ad Caelum, the guardians of light kept KH at bay by keeping it shut. Sora rendered it "inert" by closing Kingdom Hearts, preventing it from releasing light and darkness to destroy the World. The End doesn't attack by opening its keyhole so idk what Sora sealing it is gonna do
Both of them are tier 1, so that wouldn't be a problem. Nil type attacks have durability negation as well, so that could help too. Sora had faced a similar style of Danmaku with the Demon Tower and the Gummi ship treck, so that tracks. Though, considering Xehanort could warp the entirety of Scala Ad Caelum, I don't see why the X-blade couldn't create a beam that could cover the same distance, especially with Kingdom Hearts. It should have enough energy to do so. But if it's too hypothetical, sealing may work.
