It's quite possible to hurt Sephiroth. He is not the end-all char everyone thinks he is. Zack's Buster Sword pierced through him like thin paper. Cloud's Omnislash at the end of AC managed to get Sephiroth out of the fight. Should the Master Chief strike Sephiroth with the energy sword the results would be the same as in Halo. He would get sliced in half right off the bat. Then you have all of the Chief's equipment. It's a shame the guy who made this topic didn't specify any weapons for the Chief. You have the Sniper Rifle who can/will put Sephiroth out from a safe enough distance. You have the Battle Rifle, Carbine which both can keep Sephiroth busy while the Chief moves in closer. You have the Energy Sword. The Rocket Luncher, the covenant FRC, Plasmas Grenades etc. Not to mention the Chief is master of every weapon. Chances are if the Chief manages to grab hold of Sephiroth's Masamune he could teach him a thing or two about using it.
The almighty Sephiroth isn't all that mighty. If he really was as powerful as many of his supporters claim he is Cloud wouldn't have stood a chance against him from the very beginning. Then you have his psychological flaw to play around with his enemies.
I am not claiming Master Chief will win 10 out of 10, but it's possible to have the Chief win some of them.
Edit: For those who may think Master Chief is simply a guy with a deep voice in a fancy suit here are some of his enhancements:
1. Carbide ceramic ossification: advanced material grafting onto skeletal structure to make bones virtually unbreakable. Recommended coverage not to exceed 3 percent total bone mass because of significant white blood cell necrosis. Specific risk for pre- and near-post pubescent adolescents: skeletal growth spurts may cause irreparable bone pulverization.
2. Muscular enhancement injections: protein complex is injected intramuscularly to increase tissue density and decrease lactase recovery time. Risk: 5 percent of test subjects experience a fatal cardiac volume increase.
3. Catalytic thyroid implant: platinum pellet containing human growth hormone catalyst is implanted in the thyroid to boost growth of skeletal and muscle tissues. Risk: rare instances or elephantiasis. Suppressed sexual drive.
4. Occipital capillary reversal: submergence and boosted blood vessel flow beneath the rods and cones of subject’s retina. Produces a marked visual perception increase. Risk: retinal rejection and detachment. Permanent blindness.
5. Superconducting fabrication of neural dendrites: alteration of bioelectrical nerve transduction to shielded electronic transduction. Three hundred percent increase in subject’s reflexes. Anecdotal evidence of marked increase in intelligence, memory, and creativity. Risk: significant instances of Parkinson’s disease and Fletcher’s syndrome.
If you add Cortana to the mix the Chief becomes even more dangerous since she pretty much regulates and enhances his and the armor's performance.