On second thought, Primal Groudon can take on Xerneas pretty well, at least on paper. It resists Moonblast, is immune to Thunder, takes pitful damage from Psychock, so it looks like Xerneas' best shots are the unreliable Focus Blast and Grass Knot.at least that's my "wild" speculation. Neither one strictly beats the other, it depends on switching. And for that reason I don't really know that either one is claiming a throne. Maybe they've just become the two mad kings? Or maybe Xerneas is just kind of laughing from his throne while his possible greatest enemies neutralize each other.
And let's not forget Primal Kyogre can't switch into Primal Groudon unless it's sure a fire-type move is coming. Groudon can even misspredict and tank an ice beam on the switch. Freeze hax isn't even an option because of the sunlight.
Primal Groundon for king.