NO, IT'S NOT THE TANK FAULT IT'S THE HEALER!!!!
Using Calm Mind, Recover, and Stored Power, Mega Latias can serve as an effective defensive sweeper for Dragon that tears through otherwise troublesome matchups such as Normal. With two extra resistances to Psychic and Fighting, Mega Latias's Dragon / Psychic typing allows it to resist many types. This can allow it set up on Pokemon such as Mega Charizard Y, Chansey, and Mew. Furthermore, a useful Ground immunity allows Mega Latias to come in on Ground-type moves as well as avoid damage from Spikes and Toxic Spikes and the Speed drop from Sticky Web. However, Mega Latias is useless in the Dark matchup if its only attacking move is Stored Power, and while it can run Thunderbolt to not be walled by Dark-types, it then loses the ability to take on walls that run a status-inducing move. Furthermore, Mega Latias struggles to set up against teams with wallbreakers such as Diggersby, Mamoswine, and Bisharp, as they are able to switch in on a Calm Mind and threaten it out with a heavy attack. Mega Latias also struggles to set up in matchups such as Fairy and Ground, as it is scared out by super effective damage or by utility moves such as Whirlwind and Haze. Mega Altaria gives Mega Latias competition as a Mega Evolution, as it is a Dragon Dance user with more immediate power thanks to Pixilate and a typing that aids in matchups like Dark, which Mega Latias cannot excel in. Mega Latias does, however, always win against Ditto, unlike Mega Altaria, which has to bank on the Ditto not running the appropriate happiness to maximize the power of Return or Frustration. It also helps take on Double Dance Mew due to Calm Mind and its resistance to Psychic, which is something Mega Altaria cannot do.
name: Calm Mind (Dragon)
move 1: Calm Mind
move 2: Recover
move 3: Stored Power
move 4: Refresh / Thunderbolt
evs: 252 HP / 4 Def / 252 Spe
Calm Mind increases Mega Latias's power and bulk as it sets up to become a powerful sweeper. Recover helps gives Mega Latias a form of reliable recovery that it can use while it sets up with Calm Mind. Stored Power is a Psychic-type STAB attack that combines well with Calm Mind due to how naturally bulky Mega Latias is and how it gains 40 Base Power from each Calm Mind. This allows Mega Latias to beat common walls like Porygon2 and Zapdos. Refresh is used to keep status off of Mega Latias so it can beat walls such as Chansey and Cradily, which all usually carry a status-inducing move, and keep it healthy longer without having to be swapped out, since it can cure itself of status conditions. This allows Mega Latias to set up with Calm Mind and KO the walls in return with Stored Power. Thunderbolt can be run over Refresh to not be completely walled by Dark-type Pokemon. It also helps with Celesteela and Slowbro as well as common Haze users and phazers such as Mantine, Toxapex, and Skarmory. The downside to running Thunderbolt is that Mega Latias cannot get rid of status and will be put on a timer.
A Timid nature is run with maximum Speed investment to outspeed common Pokemon such as Keldeo, Mega Charizard Y, and Cobalion. 252 HP EVs are run in order to make Mega Latias as bulky as possible to offer it more setup opportunities. Levitate allows Mega Latias to come in on Ground-type attacks for free and avoid entry hazards like Toxic Spikes, Spikes, and Sticky Web. Latiasite allows Latias to Mega Evolve.
Mega Latias is a sweeper with Calm Mind and Stored Power, so it should set up mid- or late-game when its checks and counters, such as Diggersby and Excadrill, are weakened or KOed. Be wary of Pursuit users like Alolan Muk and Scizor, as they threaten to do a lot of damage, leaving Mega Latias in range for another Pokemon to KO it. Set up Mega Latias against walls that do not threaten it such as Gastrodon, Chansey, and Porygon2. Mega Latias can also set up on special attackers that do not carry super effective coverage like Zapdos, Mega Charizard Y, and Landorus. Make sure to keep Mega Latias as healthy as possible with Recover while setting up with Calm Mind. Mega Latias can be brought in on resisted attacks and on Pokemon that are Choice-locked into non-super effective attacks. Avoid Toxic if not running Refresh, as it cripples Mega Latias and can put it on a timer, preventing it from effectively setting up and sweeping. Be careful not to set up on wallbreakers that can take an attack and 2HKO Mega Latias in return, such as Mega Diancie and Diggersby.
Garchomp can aid with Stealth Rock, which wears down foes and breaks Focus Sashes so Latias can more easily sweep. Focus Sash variants of Garchomp can also take care of Pokemon such as Mega Diancie, which Mega Latias appreciates. It also aids in the Steel matchup, which Mega Latias struggles in, thanks to Fire Blast and Earthquake. Latios is the ideal Choice Scarf user for Dragon, as it helps with fast threats that Mega Latias struggles with such as Hydreigon and Blacephalon. It also provides Defog support to keep Stealth Rock off of the field for Mega Latias. Hydreigon is another option; the main way it supports Mega Latias is by threatening Steel- and Ghost-types that Mega Latias struggles to beat thanks to Fire Blast and Dark Pulse. It is a great special wallbreaker and weakens threats to Mega Latias thanks to the boost from Choice Specs. Kyurem-B is a great mixed wallbreaker with Life Orb that can be paired with Latias to help break Water and Flying teams, and it takes on Toxapex and Alolan Muk, which Mega Latias can struggle against. Kyurem-B can also run a Choice Scarf to help check Mega Altaria in the Dragon mirror. Dragonite is another good physical wallbreaker that can use Extreme Speed to snipe frail Pokemon such as Greninja. Its Fire Punch and Superpower help KO Steel- and Dark-type Pokemon such as Tyranitar, and Ferrothorn. Kommo-o is a great mixed sweeper that can deal with Steel-, Ice-, and Dark-types like Heatran, Mamoswine, and Alolan Muk that Mega Latias has trouble dealing with.
Substitute can be run to shut down any weak special attacks after a Calm Mind boost and avoid status moves such as Toxic. However, be aware that Chansey easily breaks the Substitute and can wear down Mega Latias. Ice Beam can be used also over Stored Power to hit Flying-types and opposing Dragon-type Pokemon such as Garchomp and Dragonite if Mutiscale is broken. The move also helps in the Ground matchup with by KOing Garchomp, Hippowdon, and Landorus. However, dropping Stored Power causes issues such as Ditto copying Mega Latias and hitting it with Ice Beam and Chansey walling Mega Latias.
Checks and Counters
**Dark-types**: Without Thunderbolt, Mega Latias is completely walled by Dark-types and therefore cannot do anything in the matchup. Even with Thunderbolt, it will have a hard time trying to set up and KO anything, since Alolan Muk can come in and force it to switch out.
**Dragon-types**: Physical Dragon-type moves from Pokemon such as Garchomp and Kyurem-B can KO Mega Latias easily. Latios has a chance to KO Mega Latias from full if Mega Latias hasn't set up a Calm Mind boost.
**Fairy-types**: Mega Latias has a hard time setting up with a Fairy-type around, with the likes of Mega Diancie, Choice Specs Tapu Koko, and Azumarill easily KOing Mega Latias if it doesn't have a Calm Mind set up.
**Taunt Users**: Taunt easily shuts down Mega Latias, and an unboosted Stored Power is nothing to be afraid of for Pokemon such as Heatran and Tapu Koko.
**Phazers and Haze Users**: Phazers such as Skarmory and Hippowdon can take advantage of Mega Latias's poor immediate power to get it off the field before it sets up. Mega Latias also has to watch out for Haze users such as Toxapex and Mantine, as they can get rid of its boosts and make Stored Power useless, but they have to be careful in case Mega Latias is running Thunderbolt.
**Physical Wallbeakers**: Strong physical attackers such as Mamoswine, Diggersby, and Mega Scizor can break Mega Latias and are not threatened by its unboosted attacks.