Introduction
Hello! This is my first RMT ever, and English is not my first language so I probably have a lot of grammatical errors. When I first started with competitive Pokémon, I wanted to play with Gen 7 mechanics (because I wanted Mega Evolution, Ditto, and Ash-Greninja at the same time because they are all cool) however, I didn't know anything about Teambuilding (not even playing main series games casually, my starter is the only Pokémon I use in battles) or Smogon. So after researching for a bit, I found about Tiers and an image that featured "iconic Gen 7 OU Pokémon", so I selected a few of them and looked them up on Smogon Dex (All movesets, abilities, EVs/IVs are from there, but I chose the team members).
And that's how I ended up with this team. The members have not changed since the team's creation, but their movesets and items did. This team surprisingly works for me a lot of the time. However, I have no clue if it's a balanced, bulky offense, hyper offense or what kind of team.
The Team
Ditto @ Choice Scarf
Ability: Imposter
EVs: 252 HP / 4 Atk / 252 Def
Relaxed Nature
IVs: 0 Spe
- Transform
While I was building this team for the first time, I originally included Ditto just because I like Ditto, but from a more realistic point-of-view, I thought that it would probably suck and be crap. Unlike what I thought, however, it turned out to be a great team member. While it usually struggles to kill the Pokémon that it transformed into, it is good at killing the opponent's last few Pokémon. It uses a Choice Scarf to avoid speed ties with the Pokémon it just transformed into, but this does not work if that Pokémon also has a Choice Scarf.
Lopunny-Mega @ Lopunnite
Ability: Limber
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- High Jump Kick
- Fake Out
- Return
- Ice Punch
Upon switching Lopunny in, it always is Mega Evolved and uses STAB Fake Out, High Jump Kick is used unless I switch Mega Loppuny or for X reason it will or has a high chance of failing. But if it is against the very common Landorous-T or Garchomp, Ice Punch is used. Return is part of the moveset for a rare just-in-case occasion.
Landorus-Therian @ Choice Scarf
Ability: Intimidate
EVs: 232 Atk / 68 Def / 208 Spe
Jolly Nature
- Earthquake
- U-turn
- Stone Edge
- Defog
This is, in my opinion, straightforward. Choice Scarf to compensate for Landorous-T's low Speed. Earthquake is for a strong STAB move, U-turn is for a free and safe switch to another team member (plus for some Pokémon with a weakness to Bug). Stone Edge for cover-edge (ba dumm tss). Defog because no other Pokémon on the team has entry hazard removal "capabilities". If the opponent has a Landorous-T in the field and it's likely they use Earthquake, I usually switch Landorous in for a type immunity.
Greninja-Ash @ Choice Specs
Ability: Battle Bond
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Hydro Pump
- Water Shuriken
- Dark Pulse
- Spikes
252 EVs in Special Attack for a boost in its moveset's damage category, 252 in Speed, so it's more likely to outspeed other Pokémon and the remaining in Defense to help just a little bit its worst stat. Hydro Pump for a less accurate but more powerful version of Surf. I do not use Water Shuriken before it has transformed into Ash-Greninja. While Dark Pulse does not seem very threatening, it surprisingly has OHKOed many Pokémon.
Heatran @ Steelium Z
Ability: Flash Fire
EVs: 4 Def / 252 SpA / 252 Spe
Modest Nature
IVs: 0 Atk
- Magma Storm
- Flash Cannon
- Earth Power
- Stealth Rock
Heatran is my Stealth Rock set-up Pokémon. And, according to the Smogon Dex, "The 4 Defense EVs are to take a +2 Sucker Punch from Mega Mawile at full health. Modest nature is preferred to improve damage rolls against Garchomp and Gliscor" Magma Storm is a powerful STAB and very useful against the common Ferrothorn. While Steelium Z is a good STAB Z-Move, most of the time I use it as a last-minute last resort. Finally, Earth Power can very easily OHKO opposing Heatran.
Magearna @ Assault Vest
Ability: Soul-Heart
EVs: 248 HP / 224 SpD / 36 Spe
Sassy Nature
- Volt Switch
- Fleur Cannon
- Focus Blast
- Iron Head
The only Pokémon on the team I didn't know existed before making the team! Volt Switch, besides being used for an easy switch, is good against Greninja and Tornadus. Focus Blast can hit Heatran, Ferrothorn, and Mega-Lopunny for Super Effective damage. While Magearna has a low Attack stat, Iron Head is used against Calm Mind users like Mega Alakazam, Clefable, and opposing Magearna.
-> 
To keep that idea of a Fire Breaker, I believe Victini with Choice Band is a better pick here. Thanks to its absurb 180 Base Power Fire Move V-Create, even with a mediocre base attack of 100, Victini is able to dig holes in any team lacking a good fire-resist. Moreover, Victini comes with Bolt Strike to hit Water, U-turn to keep Momentum and Trick to cripple Defensive walls such as Toxapex, Gastrodon or Heatran. 
-> 
Overall, I'm not a fan of Ditto outside of Balance/Semi-Stall as its two main assets is to give infinite PP against Stall and being a vicious revenge killer against HO/Set-Up pokemons. Therefore, I think it's better to solidy your defensive backbone by adding a Rotom-Wash instead. Rotom-Wash is a very usual pivot as it brings many greats things such as many key-resistances (water, fire, flying, ground immunity, ...) and plenty of excellent support moves (Volt-Switch, Will-o-Wisp, Defog, ...).
-> 
With the adding of Victini, I think Greninja with the ability Protrean is a better set. Thanks to the combination of Dig + Groundium-Z, Greninja can unleash once a physical 160 BP Ground Move and, thus, be able to fake a possible Greninja-Ash with Choice Specs and target the assigned checks such as Magearna Assault Vest, Toxapex or Tapu Fini (which is greatly appreciate by Victini and Lopunny-Mega).
While the speed control is always a good thing, I believe Lopunny-Mega alongside Magearna and Rotom-Wash will be enough on that department. Therefore, I will go for a much bulkier variant with Leftovers instead of Choice Scarf. The main reason of that pick is that Landorus-Therian is one of the better setter of Stealth Rock in my opinion, especially when this latter can punish common defogger such as Rotom-Wash or Tornadus-Therian with the move Toxic, which, again, is appreciate by Victini and Lopunny-Mega.