The only reason I can see them fully scaling to the Meteor is cause they can fight with Rayquaza who easily destroyed the meteor and fought the person who was controlling it... but scaling them to Rayquaza is kind of dubious since he is seen as above them.
but if we cut the AP in half then Groudon has an advantage of 5.8x which can be further increased with his stat increase moves and stat decreasing moves he can use on Mira
How does Mira's Yeopo Bongseon and Direct Contract work? they don't have a given value just "they're stronger" so i'm not sure they can really cover Groudon constantly increasing his own stats well decreasing her own
as for the heat thing, Groudon's existence itself caused
the suns heat to increase drastically which
threatened to end all life on earth and should be vastly superior to any other regular heat based mon like
Magcargo or
Camerupt, both of whom have statements of heat that just sits within them at 18,000 degrees fahrenheit. as for the Range thing, he has statements of being able to expand his heat on almost a Continental to Planetary scale "Said to have expanded the lands by evaporating water with raging heat. It battled titanically with Kyogre." and that him simply existing caused the heat for the entire world to increase drastically, to the point it'd kill everything.
Mira has no heat resistance on the profile (at the moment) so its not something that can be used, meaning Groudon's passive heat is gonna slow her down either way.
all of Groudon's attacks have massive AOE to them and against Mira who is gonna be standing on the ground, all of his ground moves are gonna be far more deadly than usual like PB or Earthquake. Groudon isn't stupid, Pokemon are incredibly smart in combat esp those that are fully evolved like Machamp or Alakazam, Mons who Groudon should be superior to. as for Groudon's moves, Precipice Blades
spikes up outta the ground trapping the opponent before exploding in their face, he can create
dozens of PB's all at once and the large
AOE explosions, even a
single one can
make and all of Groundon's
Beams based moves have the same
byproduct. so Mira is gonna have to hyper focus on avoiding a MASSIVE amount of projectiles being thrown at her from Thousands of Kilometers away all well getting her Stats decreased by moves like Scary Face or Mud Shot, well also increasing his own stats with
Bulk Up which increases his strength and
Ancient Power can boost all the users stats.
Groudon also has passive healing due to its ability meaning he'll constantly recover his HP while standing on lava/magma, soil, or sand. lava and magma being things hes constantly creating from his body and standing on means he'll be recovering from any damage he could take.
Thats not to doubt Mira, Groudon isn't stupid but Mira is a combatant
Genius and similar foes have dodged PB before i.e Alain's Mega Charizard tho even that eventually got caught off guard by one.
her
Supernatural Willpower and Extreme Tolerance to Pain means she could likely likely fight through the pain tho eventually that 5.8x difference is gonna be hard to ignore esp when it grows through Groudon using certain moves.
her Analytical Prediction and information analysis that comes from fighting means she could make up for having her speed being lowered and possibly help her dodge Groudon's onslaught of attacks.
but i'm currently leaning towards Groudon tbh his ap advantage is gonna be hard to cover esp given his onslaught of projectiles he'll be throwing out which could take even the most mobile of pokemon off guard and trap them before exploding in their face, she also has to worry about his passive heat emitting which should be above that of other mons who casually sit at 18,000 degrees fahrenheit and his heat does emit since his main way of drying out the oceans was through his own heat or increasing the sun's own heat simply by existing.
I'll wait to see what David says about Mira tho since I know nothing about this character beyond what her profile says.