Red Ronin did fight Marvel Godzilla to a standstill more then once, HOLDING BACK, and outright killed a King Kong ripoff that had an edge on Godzilla. He has a beam sword that can cut through just about anything, can survive Godzillas radioactive breath, has a shield that can outright block it and also be thrown like Cap's shield AND is computer guided to boot. He also has a beam gun, but the only thing it's really done is terminally damage a heli-carrier and bring down a rockslide..
__________________ Quotes from Hia8:
"I claimed that the science is sometimes faulty."
"You don't understand. This is fiction. That means none of this stuff really happened."
"There is no writer to purposely ignore a character's natural ability just because it suits the story."
"in some cases because the writer knows that Character A will dominate Character B easily and refuses to allow this to happen for the sake of the story."
All you really need to know, is he held his own against Godzilla when his pilot was expressly trying NOT to harm him. He solar sword can, and has, injured Godzilla, and he's also physically restrained him. And, he's killed another monster that was beating Godzilla, and managed to seriously injure him.
And keep in mind this same Godzilla gave the Avengers problems, including Thor.
Personally, I thought Red Ronin was a pretty interesting foil for Godzilla.. Even if the kid that piloted him was annoying as hell.
Also, it's not just that he held his own against Godzilla, he was CREATED to combat Godzilla. With the assistance of Stark International to boot, although Stark himself didn't design it.
And this is one "Godzilla buster" that actually worked, matching Godzilla at every turn, and often outmatching him. If the kid piloting it wanted to, he could have easily wiped the floor with him. But the last thing he wanted to do was harm Godzilla. He was trying to get S.H.I.E.L.D. off his back so he can live in peace.
IMO, given what it's designed to combat and how it performed, I think it would have good odds against Fin Fang Foom.