The only reason Octavia won is because she used her head and outsmarted Luna. It became about more than just the physical side of things.
Luna was also lucky to defeat Roan in the sense that the black rain arrived at the right moment, and she's immune to it being a natblida. Which raises another point for me: how is it a fair fight when she is immune to the rain and the remaining fighters are not? It's an unfair advantage, which I suppose cannot really be helped.