Dio Brando would get fucked.
DIO could give most heroes a run for their money. Six seconds of time stop and The World punching your shit in is no joke.
DIO fully merged with Jonathan's body would need to be obliterated within minutes of it happening, since the amount of time The World can freeze begins increasing exponentially. Unless you're someone crazy powerful like Dr. Strange and either activate some time fuckery before DIO freezes time, or has a way to circumvent it, he's impossible to defeat.