I'm not the most experienced battler, but I'll try to answer if no one else will.
Magearna
Assault Vest Magearna isn't bad, but your team lacks a setup sweeper, and since your team is hyper offense, the special defense gained from Assault Vest probably isn't worth the speed boost Shift Gear.
Magearna @ Fairium Z
Ability: Soul-Heart
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Shift Gear
- Fleur Cannon
- Thunderbolt
- Focus Blast
Shift Gear boost Magearna's speed by 2, giving it the abilility to sweep. Fleur Cannon is its most powerful STAB attack, and Fairium-Z makes it even more threatening, while also preventing a crippling special attack drop. Thunderbolt hits bulky water types like Toxapex, and Focus Blast hits steel types.
Landorus-T
Defensive Landorus-T is fine, but you might want to consider a Focus Sash lead instead. Your team is hyper offense, so I think a suicide lead might be a better way to set up Stealth Rocks.
Landorus-Therian @ Focus Sash
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Stealth Rock
- Earthquake
- Explosion
- Swords Dance
This is the Pokémon you should lead with. Set up Stealth Rocks. Swords Dance can punish passive Pokémon, or Pokémon that will try to Defog away your Stealth Rocks. Earthquake is for STAB. Explosion deals huge damage, and lets itself faint. This gives you the opportunity to bring in one of your fast attackers safely.
Ash-Greninja
Your Ash Greninja is mostly fine, but I would recommend replacing Ice Beam with Spikes. You may not think Spikes makes sense on a Choice Specs set, but it is a great way of punishing Toxapex switch-ins. Spikes is very helpful, as it can weaken opposing Pokémon, making it easier for your Pokémon to sweep and/or clean late game.
Heatran --> Kartana
I feel like Magearna uses the Z-move slot better than Heatran does for your team, and you could use another late game cleaner. You also seem to have more special attackers than physical attackers. Therefore, I think you should replace Heatran with Choice Scarf Kartana. Although you miss out on Heatran's ability to check fire types, I think it's fine considering the amount of pressure Ash-Greninja puts on fire types. Kartana also benefits from Landorus-T's ability to weaken opposing Pokémon early game.
Kartana @ Choice Scarf
Ability: Beast Boost
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Leaf Blade
- Sacred Sword
- Smart Strike
- Knock Off
This Pokémon should generally be used as a late game cleaner. Leaf Blade is its most powerful STAB attack, hitting Pokémon that don't resist it extremely hard. Sacred Sword allows it to hit steel types like Heatran and Ferrothorn. Sacred Sword is another STAB move, which is super effective on fairy types like Tapu Bulu and Clefable. Knock Off can be used to cripple Pokémon such as Chansey, Toxapex, Heatran, and Celesteela, which depend on their held items a lot.
Latios
Your Latios is mostly fine. I would replace Thundrbolt with Defog, as your team lacks a reliable way to clear hazards from the field. I would also replace its Life Orb with Soul Dew because you it doesn't have any recoil damage, and all of your attacking moves are boosted by it. Watch out for pursuit trappers like Tyranitar.
Mega Lopunny
Your Lopunny is mostly fine, although you may want to consider replacing its Fake Out or Ice Punch with Power-up Punch or Encore to defeat bulkier Pokémon.
Here is your team with after all the changes I made. You don't have to take all of my suggestions. The changes made are in bold.
Magearna @ Fairium Z
Ability: Soul-Heart
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Shift Gear
- Fleur Cannon
- Thunderbolt
- Focus Blast
Landorus-Therian @ Focus Sash
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Stealth Rock
- Earthquake
- Explosion
- Swords Dance
Greninja-Ash @ Choice Specs
Ability: Battle Bond
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Water Shuriken
- Dark Pulse
- Spikes
- Hydro Pump
Kartana @ Choice Scarf
Ability: Beast Boost
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Leaf Blade
- Sacred Sword
- Smart Strike
- Knock Off
Latios @ Soul Dew
Ability: Levitate
Shiny: Yes
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Roost
- Defog
- Psyshock
- Draco Meteor
Lopunny-Mega (M) @ Lopunnite
Ability: Limber
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Power-Up Punch
- Return
- Ice Punch / Encore / Fake Out
- High Jump Kick
If you want more advice on how to utilize your team, go to Smogon's Strategy Pokedex, and look at the Usage Tips they give for each of your Pokémon. Good luck!
