She can just use her reality warping to alter her strength to make herself stronger. For whatever reason Goku will always let his opponents reach their full power, no matter if they are good or evil. Plus, since he body is pretty much just a projection of herself. Dove right into the galaxy cauldren, and managed to force Chaos out, but spared it. Once the smoke cleared Usagi and Goku were still both standing, with Vegeta, as well as Usagi's friends to be shocked. That is why she became Sailor Cosmos.