The Red Legion are a miniscule force compared to the Covenant. The Covenant has them beat by ship numbers alone. This is like asking "who would win, the 1st Infantry Division or the entire Chinese military?". It doesn't matter that 1ID is better trained and equipped, they simply don't have the numbers or weapons necessary to fight off an endless horde of similarly-but-not-quite-as-well-equipped-and-trained soldiers and other military millieu