I hope you're joking. Stark's suit does have a shield. It took a nuke at 2% power. Also, he has class 100 strength, meaning he can lift 100 tons or greater. Probably greater. He's already powerful enough to tear off John's suit like a tin can (and that's in his normal suit), but he's also been trained by Captain America, meaning he's probably just as skilled as John too. Also, any one of his weapons would punch right through John's puny shield like nothing. A repulsor round would send John flying; a uni-beam would instantly put a hole through him. John has literally no chance here.

No, here's what happens in a fight. Master Chief runs off into a UNSC arms depot and hyperamps his jetpack with help from Cortana. Chief uses his infinite jetpack to fly to the Ark. Chief activates all the rings and kills Stark.

One small problem........ Cortana's dead.