Latias (Dragon):
Standalone offensive supporter (no Latios as a teammate).
Latias (F) @ Life Orb
Ability: Levitate
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Psyshock
- Draco Meteor
- Defog
- Healing Wish
Alternate items: Latiasite over Life Orb.
Latiasite provides very nice bulk enchancement in cost of slight amount of power: don't expect Draco Meteor to dish out lots of damage. Therefore I personally suggest using Life Orb on standalone Latias (without Latios as a teammate).
Alternate moves: Tailwind over any of these moves.
Tailwind is a great supportive move for somewhat sluggish Dragon-types like Mega Garchomp, Dragonite and non-Choice Scarf Black Kyurem, but putting it into the set is a challenge. From my experience, it's best used over Defog if you don't mind your Rock-weak Pokemon having their health cut, but Tailwind followed by Healing Wish makes a huge momentum shift in your favour.
Alternate spread: 72 HP / 184 SpA / 252 Spe
Only if not running Latiasite, and I don't really think Monotype Latias needs this OU Latias' spread, since it's not going to check Thundurus often, and offensive Latias is better having all the power she can get.
Latios-supported offensive Defogger.
Latias-Mega (F) @ Latiasite
Ability: Levitate
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk / 30 SpA / 30 Spe
- Energy Ball
- Hidden Power [Fire]
- Defog
- Healing Wish
Alternate items: Expert Belt or Leftovers over Latiasite.
Latiasite is more suggested on this set, but Leftovers or Expert Belt could be used if your Mega slot is taken already. Expert Belt is preffered as a damage boosting item over Life Orb because this set focuses on landing supereffective hits.
Alternate moves: Thunderbolt + Ice Beam over Energy Ball + Hidden Power Fire.
If you have both Latias and Latios in your team, it's best to let Latios wallbreak with Psyshock and Draco Meteor, while Latias will take on something that Latios cannot just OHKO. Therefore, Latias can forfeit Psyshock and Draco Meteor and focus on using one of two dual-move offensive combinations to attain the maximum coverage.
Energy Ball + Hidden Power Fire is the first option, and I'm more keen of it because it goes very well with Latias' primary job: to Defog the hazards away, while the majority of hazard setters are weak to either Fire or Grass and are outspeeded by Latias, as well as lured to the false sense of security: both Lati twins share the same checks, and if Latias eliminates them, Latios will have easier time breaking stuff. It also lets Latios not to run Hidden Power Fire and preserve his valuable 350 Speed. Preferred coverage moves for Latios: Ice Beam, Surf, Thunderbolt, Shadow Ball.
Thunderbolt + Ice Beam is an another one. It threatens less number of hazard setters compared to the first option, but gains equally decent coverage outside of that. Also note that it's effective on a different spectra of Pokemon. Preferred coverage moves for Latios: Energy Ball, Hidden Power Fire, Shadow Ball.
Bulky supporter.
Latias-Mega (F) @ Latiasite
Ability: Levitate
EVs: 252 HP / 4 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Psyshock
- Dragon Pulse
- Thunder Wave
- Roost
Alternate nature: Bold.
Depending on your EV spread (look below).
Alternate moves: Energy Ball + Hidden Power Fire or Thunderbolt + Ice Beam over Psyshock + Dragon Pulse, Surf or Thunderbolt or Hidden Power Fire or Hidden Power Fighting over Dragon Pulse. Defog over Thunder Wave.
The choice of first three moves largely depends on what your team needs, because Latias' movepool is really vast. Psyshock + Dragon Pulse is a default combination and can be adjusted in many ways. If you want Mega Latias to check Fighting-types, use Psyshock + a neutral or effective move to Steel. If Fighting-types are not of your concern, either Energy Ball + Hidden Power Fire or Thunderbolt + Ice Beam are for you. Thunder Wave can cripple a lot of Pokemon who threaten Dragon teams, but if it's not needed, Defog is a good alternative. I also would like to note that mono-attacking Latias is underwhelming in Monotype no matter what set does it run.
Alternate spreads: 252 HP / 228 Def / 28 Spe, 252 HP / 124 Def / 132 Spe
Only for Thunder Wave sets. Both of these options alongside with Bold nature make Mega Latias a really sturdy tank. The former spread outspeeds unboosted Cloyster with a Speed-increasing nature, Breloom, Bisharp and standart specially defensive Zapdos, as well as evading OHKO from Choice Scarf Garchomp's and Salamence's Outrage. The latter one outpaces non-Choice Scarf Unbound Hoopa, unevolved Mega Gardevoir, Mega Gyarados and 288 Speed Mew, but it gives Garchomp and Salamence the chance to OHKO Mega Latias with Outrage. Mega Latias can hit a lot of different benchmarks, but these are the most important.
Calm Mind.
Latias-Mega @ Latiasite
Ability: Levitate
EVs: 252 HP / 228 Def / 28 Spe
Bold Nature
IVs: 0 Atk
- Psyshock
- Dragon Pulse
- Calm Mind
- Roost
Alternate nature: Timid.
For the alternate EV spread (look below).
Alternate moves: Energy Ball + Hidden Power Fire or Thunderbolt + Ice Beam over Psyshock + Dragon Pulse, Surf or Thunderbolt or Hidden Power Fire or Hidden Power Fighting over Dragon Pulse.
Same as with Bulky supporter set, except the fact Psyshock is more valuable in this set to stirke both physically and specially bulky Pokemon.
Stored Power as a lone attacking move is not listed in this set for a reason. Before such Mega Latias gains at least two Calm Mind boosts she has near to no offensive presence - something totally unfitting for a Dragon team. In addition, having a Pokemon who is entirely useless in battles against Dark teams is a bad idea as well. Even if Stored Power is run alongside another attacking move, it is still mostly inferior to Psyshock due to necessity of having Calm Mind boosts and inability to threaten special walls.
Alternate spread: 252 HP / 4 SpA / 252 Spe.
A faster, but much less bulkier Mega Latias.
Dual Screens.
Latias (F) @ Light Clay
Ability: Levitate
EVs: 252 HP / 4 Def / 252 Spe
Timid Nature
IVs: 0 Atk
- Reflect
- Light Screen
- Defog
- Healing Wish
Alternate moves: Psychic or Psyshock or Tailwind over Defog.
Psychic or Psyshock increase Latias' offensive presence from zero and let you do damage when you don't want to switch your Pokemon, while Tailwind gives your wallbreakers a chance to ravage an entire opponent's team, and as it was noted before, it's especially good alongside Healing Wish. Make sure you have an alternative Defog user if you opt to not have it on Latias!
P.S. please, fix Dragon Hydreigon Choice Scarf set to this:
Hydreigon @ Choice Scarf
Ability: Levitate
EVs: 4 Atk / 252 SpA / 252 Spe
Naive Nature
- Dark Pulse
- Draco Meteor
- Fire Blast
- U-turn
Alternate spreads: Hasty Nature has a better time tanking special hits, while Naive one lets you take most of priority moves better. The choice is up for you.
Alternate moves: Flash Cannon or Earth Power over Fire Blast. Make sure to have a Fairy check, though! Earth Power is a somewhat compromise between Fire Blast and Flash Cannon, as it hits both Steel-types and Mega Diancie.