This fight is going to have two phases:
Phase 1 is going to be a massive beam spamming war where beams are going to be flying all over the place. Obviously, Super Mechagodzilla is going to be king of this phase since he won't take damgage from the beams, instead, turning them into his plasma grenade. However, the other monsters have all survived some pretty powerful beams before; so, they are going to scrape by through Phase 1.
At some point, some of the monsters are gonna change tactics and start using their non beam attacks, which will begin Phase 2. I imagine that the pilot of Mecha King Ghidorah will use the Godzilla Capture Cables to electrocute SMG which will hurt his internal systems. However, SMG might return the favor with his cables or just ovewhelm MKG with fire power. Either way, if Mechagodzilla is lifted into the air by MKG and then dropped, the damage will be devastating to both machines. All in all, I don't see either one making it out.
Meanwhile, Legion, Leo, and Orochi are gonna be duking it out. Now, at first, Leo will have the aerial advantage, thus avoiding most damage while still dishing out some extra helpings of beam spamming. However, if at any point, Legion loses her horns, it will be rather easy for her to take out Orochi and Leo with the laser whips since they aren't as durable as Heisei Gamera, who fell to the same attack. And, it would take a ton of Leo's fire power to equal the power of the Mana Cannon, so Legion will probably survive the Leo beam spam.
So, Legion takes the most victories, although I could see the other monsters having fighting chances.
If you have nothing to lose, than you have nothing to fight for.