On the ground Star Wars already has the advantage. The fighting tactics of Spartans are something that would hold the field, but they are faced with tragic flaws: they're still using lead based weapons, and they are few in numbers. Their high powered shields are vulnerable to energy based weapons, which is the only weapon class in Star Wars. But if the UNSC and the Covenant are working together than the Spartans would survive longer. Brutes, Skirmishers, Jackals, Grunts, and the UNSC marines would most likely be facing the clones, Stormtroopers, rebels, and the droid army. The droids would drop out quickly from simple over charged plasma pistols. And the marines and rebels would have gruesome battles eventually ending in a stale mate. Brutes, and the Covenants 3 species of infantry would last a long time against the 2 great clone armies, but that's just it, they're facing down clones. The Halo aliens' numbers would slowly dwindle as they normally reproduce, while the clones are constantly being replenished.
So then it would come down to some endless fights between Hunters and drones vs. the most skilled clones and any adaptive droids. The clones and droids would first attack the hunters, thinking that the drones will run once their giant friends are dead. The drones would constantly ambush the distracted Star Wars forces, but the clones would still keep coming. It would all bleed down to whose resources will be exhausted first.
Up in space fighters would have endless battles, while the frigate class ships would slowly kill each other. But during this time, the relentless Covenant would be glassing the battle grounds, ignoring collateral damage. But the Empire and Republic would make short work of that.
When the flood comes into play, and the Empire's zombie grow in numbers, battle worn planets would be obliterated either by glassing or being owned by the Death Star. Overall the flood and zombies would gain an upper hand for a bit but they would eventually be snuffed out.
Finally it comes to the last battles, held by Elites and Spartans vs. Clone Commandos and Jedi/Sith. The honor driven Elites would pull out their energy swords and dual any Jedi/Sith. The Sith would go crazy with their force powers and destroy the Elites, while the Jedi would respect their disadvantage and not use the Force. Overall the Elites would suffer heavy loses. But on the other hand the more driven and skilled Spartans would probably defeat the commandos but with a good chunk taken out of their numbers. So then there would would be a large group of Jedi/Sith and a ragtag team of amazingly skilled Elites (or some that were smart enough not to get in a sword fight with someone that can "throw lighning at them) and the Spartans that include the Master Chief and any as skilled as him.
Sorry Halo fans but I think the Jedi and Sith would win. It's only because they have Force powers, with out it they would lose. But wait there's more! The Forerunners! (applause XD)They would come in (late as usual) and try to wipe up the mess.
Personally, I think Star Wars would be put into a stale mate. And before you go, "BOOOO! You're only being neutral, so no one will get ma at you!", define stale mate; one player can't make a move without becoming in danger. A.K.A: they're trapped, and will die of starvation (in reality).
So Halo sort of wins. But that's because they have all powerful beings on their side. Without the Forerunners Star Wars would win.