This is based off a pokepasta I forgot the name of where Groudon wins against Brandon and turns the entire world into a massive volcano.
I'm guessing he would stay somewhat alive by a combination of Aqua Ring and other water-type moves from Marshstomp, though both would technically be dead. Several body parts melted off, scalding him like wax, as he wandered the volcano, looking for an escape or end to the heat. At least, I think that's what would happen after Groudon's takeover of the world.