Landorus with Earth Power is my personal counter to Primal Groudon. It’s faster, and can OHKO a full HP/Special Defense Primal-Groudon
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This will always, always counter Primal Groudon
Actually there are probably a few other legendaries that I know can learn Earth Power that can take out Groudon. Giratina, Palkia, Dialgia, Arceus, Heatran, Ho-Oh/Lugia. Diancie and Volcanion.