Overall, this team is pretty okay, it just has some moveset and item issues.
Blacephalon, Mega-Garchomp and Zeraora are very mediocre choices in the format, and can be replaced with better Pokemon.
Blacephalon --> Heatran
Heatran @ Leftovers
Ability: Flash Fire
EVs: 252 HP / 176 SpD / 80 Spe
Calm Nature
- Lava Plume
- Toxic
- Protect
- Earth Power
Blacephalon is not that good. It's weak to Stealth Locks, extremely frail which means it barely gets time to set-up Calm Mind. Meanwhile, Heatran is a very good Fire-type, not to say the best in Gen 7 OU. It's defensively able to check stuff like Magearna, Mega Mawile, Tornadus-T, and Ferrothorn.
Lava Plume has 30% chance to burn, which is great to abuse vs physical attackers. It has decent PP as well. Toxic is able to pressure most Pokemon, and the combination of Earth Power + Lava Plume + Toxic has few absorbers. Protect is for scouting the opponent's move, meanwhile Earth Power covers other Fire-types like Victini, Heatran, etc.
Zeraora --> Rotom-Wash
Rotom-Wash @ Leftovers
Ability: Levitate
EVs: 252 HP / 200 Def / 56 Spe
Bold Nature
- Hydro Pump
- Volt Switch
- Will-O-Wisp
- Defog
Zeraora was UU in Gen 7 and wasn't even ranked in the viability rankings. A much better Electric-type can be used in form of Tapu Koko offensively or Rotom-W defensively, but since we don't have a Defogger the latter option should be chosen.
Greninja-Ash
Greninja-Ash (M) @ Choice Specs
Ability: Battle Bond
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Dark Pulse
- Water Shuriken
- Ice Beam
- Hydro Pump
The set is pretty fine, but why Rocky Helmet? First, Greninja-Ash is very weak physically; it just hits and rips without tanking hits. Rocky Helm on a special sweeper is astroshes just like my English. Outside of that, it's pretty fine but I prefer Hydro Pump over Surf because I like power over accuracy; though it's all of your preference.
Garchomp-Mega --> Landorus-Therian
Landorus-Therian (M) @ Flyinium Z
Ability: Intimidate
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Stealth Rock
- Swords Dance
- Earthquake
- Fly
Mega-Garchomp is... eh. That thing actually loses speed after mega-evolving, which is really bad. It isn't that great in Gen 7 OU as a Mega, why not use Landorus-T as your Ground-type? Better ability, can hold a moveslot actually makes it a lot better.
This is our Stealth Rocks setter, and Z-move abuser. Hazards are very important indeed. Fly is STAB, but it becomes a one turn nuke Super Sonic Strike. Earthquake is standard STAB, and things.
Kartana
Kartana @ Choice Band
Ability: Beast Boost
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Leaf Blade
- Knock Off
- Sacred Sword
- Smart Strike
Kartana's Steelium set can easily be blocked by Zapdos. We also have Flyinium crystal on Landorus-T as our boosted breaker, so why not a raw wallbreaker?
Choice Band Kartana is a fantastic breaker, it's able to nearly 2HKO the whole meta thanks to its coverage. Leaf Blade breaks through Toxapex, Garchomp, etc. and things with ease. Knock Off can remove items from checks like Zapdos to make breaking through them easier. Sacred Sword is for Ferrothorn, Heatran, etc. OHKOing/2HKOing them. Smart Strike hits Clefable, Mega-Venusaur, etc. that can annoy Kartana.
Magearna
Magearna @ Assault Vest
Ability: Soul-Heart
EVs: 248 HP / 224 SpD / 36 Spe
Sassy Nature
- Volt Switch
- Fleur Cannon
- Focus Blast
- Iron Head
You don't need dual STAB. Your set gets walled by Heatran, Toxapex, and most other steel-types pretty easily. Our team has special Greninja as for breaking, so how about a Defensive approach? We had really appreciate a Greninja, Clefable, Reuniclus check and Magearna can deal with those. Volt Switch is for pivoting and can bring in Specs Greninja easily. Fleur Cannon is strong STAB, Focus Blast covers steels while Iron Head deals with Calm Mind Clefable that can tank other hits easily.
Tl;dr
Full Team
Hope this helps!! I haven't done OU RMTs much and I am fairly unknown with the meta, so take everything with a pinch of salt :)
