Sion lasts a second deflecting fire, then his lightsaber gets shot out of his hand and destroyed. Once the 501st runs out of ammo after a few hours, sion gets up in another hour, and has, maybe 15 minutes to kill thousands with his hands. He loses, and gets his arse kicked.
Actually, now that I think about it, they wouldn't run out of ammo. Sion loses, no matter how you slice it.