Mega Heracross Weak List (Bold is 1HKO guaranteed, slanted is 1HKO possible. Only to B+ tier for rares, A for mega,A for common.)
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Mega Rayquaza (OHKO Dragon Ascent/Hurricane), Eternatus (2HKO Eternabeam/Flamethrower), Primal Groudon (OHKO Eruption), Primal Kyogre (OHKO Water Spout), Victini (OHKO V-Create/Overheat), Yveltal (OHKO Sky Attack/Oblivion Wing), Mega Diancie (OHKO Moonblast), Magearna (OHKO Fleur Cannon), Xerneas (OHKO Moonblast), Zygarde-Complete (2HKO Outrage), Giratina (2/3HKO Shadow Force (+ Shadow Sneak if outsped by Heracross)), Zekrom (2HKO Bolt Strike/Fusion Bolt/Outrage if outspeed), Solgaleo (2HKO Zen Headbutt), Marshadow (3HKO Spectral Thief), Zacian (2HKO Moonblast), Galar-Zapdos (OHKO Drill Peck)
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Mega Mewtwo (X & Y) (OHKO Future Sight), Mega Metagross (OHKO Zen Headbutt), Mega Gengar (OHKO Dream Eater), Mega Aerodactyl (OHKO Sky Drop), Mega Blaziken (OHKO Flare Blitz/Brave Bird), Mega Gardevoir (OHKO Future Sight), Mega Salamence (OHKO Fly), Mega Gallade (2HKO Psycho Cut (Requires 26+ Def and 29+ HP))
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Metagross (2HKO Zen Headbutt + Bullet Punch), Braviary (OHKO Brave Bird/Sky Drop), Primarina (1/2HKO Moonblast (OHKO Modest or >25 Sp. Atk Timid, 2HO <26 Sp. Atk Timid)), Dragonite (2HKO Outrage/Fire Punch), Salamence (OHKO Fly), Delphox (OHKO Future Sight), Hawlucha (OHKO Sky Attack), Arcanine (OHKO Flare Blitz), Gardevoir (If Outspeed, OHKO Future Sight/Dream Eater), Darmanitan (OHKO Flare Blitz), Archeops (OHKO Wing Attack/Pluck), Chandelure (OHKO Inferno/Overheat), Toxapex (Wins via 1X-HKO Venom Drench, Recover, Liquidation/Poison Jab), Kommo-o (2HKO Close Combat)