Decided to make a team centered entirely around Mega Gyarados, so here ya go. Feel free to let me know if I should change anything.
M-Gyarados @ Gyaradosite
Ability: Mold Breaker
EVs: 252 Atk/ 252 Spe/ 4 SpD
Jolly Nature
Moves:
Dragon Dance
Waterfall
Crunch
Earthquake
This is the set all the cool kids run. Dragon Dance to just increase it's stats and to make it a more scary sweeper. Waterfall as it's primary STAB attack. Also for that scummy flinch chance. Crunch as another STAB attack, as it's defense drop can also potentially help it against more physical mons. Earthquake as coverage against a couple others. Also to hit some popular types such as Steel and Electric more effectively. This is the primary sweeper of the team. It's main goal is to set up some DD's and either deal a serious amount to things or downright ko'ing them.
Landorus-Therian @ Yache Berry
Ability: Intimidate
EVs: 149 HP/ 229 Def/ 30 SpD/ 100 Spe
Impish Nature
Moves:
Stealth Rock
Earthquake
Toxic
U-turn
I still hate this thing alot. So the Yache Berry is used so it won't get OHKO'ed by a (normal) Greninja and a bulky Mew. Also to live and HP Ice from Koko. This also let's it so Mamoswine never KO's it with Icicle Crash. Yes, I know that's situational but bear with me. It can use Earthquake as it's main stab attack. As even an Earthquake with no attack investments still allows it to still hit pretty hard. Toxic to wear down Tangrowth and Zapdos, the ones that threaten this set the most. U-turn to gain momentum, that's it. Landorus serves as the main Stealth Rocker on the team that also puts a dent in most things with it rather high attack stat.
Toxapex @ Eject Button
Ability: Regenerator
EVs: 252 HP/ 192 Def/ 64 SpD
Bold Nature
Moves:
Toxic Spikes
Haze
Recover
Infestation
It's Eject button is used for it to absorb any hit. Even it puts a dent in Toxapex, it'll always get the regenerator heal. It's also used so I won't let any other mon on my team to take a seriously hard hit. Toxic Spikes to wear grounded foes (that don't have magic guard and aren't part steel or poison or flying). The poison can also help it out with Mega Gyarados as it let's it stand a chance against Tangrowth (seriously why is Tangrowth such a problem to me sometimes). Haze to get rid of anyone trying to set up on it. Because those 63/52 offenses are definitely threatening to everything. Recoverto piss people off to let it heal much more reliably and let it become more spammable. Infestation to trap anything that a) can't do too much to toxapex and b) allows it trap foes while letting them take more damage overtime. Toxapex is used for sponging hits, walling certain mons, and setting up the broken ass Toxic Spikes.
Magearna @ Steelium Z
Ability: Soul Heart
EVs: 252 Spa/ 252 Spe/ 4 SpD
Timid Nature
Moves:
Shift Gear
Flash Cannon
Aura Sphere
Thunderbolt
This is meant to be the special wallbreaker. Or something like that. Shift gear to double it's trash speed. It allows it to compete with outspeed some fast threats such as Ash-Greninja, Tapu Koko, and others. Flash Cannon is used as it's main STAB attack. Pair it up with the Steelium Z, and you get something that let's you put a dent in things that aren't named Chansey. Aura Sphere put together with Flash Cannon gives you pretty good coverage on alot of things. I was going to run Focus Blast, but that 70% accuracy was so good, I didn't think I was worthy, so I put Aura Sphere instead. Thunderbolt also allows it too hit Toxapex, Zard Y, Gyarados, and non Lightning Rod Alolan Marowak's. Those were the mons that didn't take too much from either Flash Cannon or Aura Sphere. This set combined with Soul Heart, allows magearna to be an effective sweeper.
Zygarde @ Leftovers
Ability: Aura Break
EVs: 240 HP/ 116 Def/ 152 SpD
Careful Nature
Moves:
Dragon Dance
Protect
Thousand Arrows
Substitute
Zygarde is meant to be made as bulky as possible. It has dragon dance to let it boost it's speed and attack. It's meant too use DD behind the sub, so it'll evade the hit so it can boost it's stats at once. It also lets it boost it's speed so it can outspeed a lot of mons in the tier at +2 and even at +1. Protect to scout out for hits and to let it gain leftovers recovery. Thousand arrows to let it hit a lot of things pretty hard that aren't Grass, Bug, or Shedinja. It also lets it hit some mons that would otherwise wall it such as Lando, Skarmory, and Celesteela. This Zygarde can either be a sweeper and/or a staller. It can wear a lot of mons down very fast. +1 thousand arrows hits alot of the tier pretty hard as well.
Gengar @ Choice Scarf
Ability: Cursed Body
EVs: 252 Spa/ 252 Spe/ 4 SpD
Timid Nature
Moves:
Trick
Shadow Ball
Sludge Wave
Focus Blast
Gengar's Choice Scarf allows it to outspeed fast mons such as Mega Alakazam, Greninja and Tapu Koko without a Choice Scarf. and some other things. Shadow Ball is it's primary STAB move. It also takes advantage at the fact of how little of the tier resists ghost type attacks. Pair it up with Focus Blast, and you get perfect coverage of the tier. It also allows it to OHKO's Tyranitar, Bisharp, and other dark types on the switch. They all would otherwise threaten Gengar out. Sludge Wave let's it hit fairy and grass types for super effective damage. It also puts a dent in a couple of other things. Trick is used to punish bulky mons on the switch. Chansey and specially defensive Toxapex would wall this set. However, Trick can give them a Choice Scarf, which significantly hinders their use in the battle and allows Gengar to beat them. This set is used to capitalize on how Choice Scarf can allow it to beat many threats in the tier.
M-Gyarados @ Gyaradosite
Ability: Mold Breaker
EVs: 252 Atk/ 252 Spe/ 4 SpD
Jolly Nature
Moves:
Dragon Dance
Waterfall
Crunch
Earthquake
This is the set all the cool kids run. Dragon Dance to just increase it's stats and to make it a more scary sweeper. Waterfall as it's primary STAB attack. Also for that scummy flinch chance. Crunch as another STAB attack, as it's defense drop can also potentially help it against more physical mons. Earthquake as coverage against a couple others. Also to hit some popular types such as Steel and Electric more effectively. This is the primary sweeper of the team. It's main goal is to set up some DD's and either deal a serious amount to things or downright ko'ing them.
Landorus-Therian @ Yache Berry
Ability: Intimidate
EVs: 149 HP/ 229 Def/ 30 SpD/ 100 Spe
Impish Nature
Moves:
Stealth Rock
Earthquake
Toxic
U-turn
I still hate this thing alot. So the Yache Berry is used so it won't get OHKO'ed by a (normal) Greninja and a bulky Mew. Also to live and HP Ice from Koko. This also let's it so Mamoswine never KO's it with Icicle Crash. Yes, I know that's situational but bear with me. It can use Earthquake as it's main stab attack. As even an Earthquake with no attack investments still allows it to still hit pretty hard. Toxic to wear down Tangrowth and Zapdos, the ones that threaten this set the most. U-turn to gain momentum, that's it. Landorus serves as the main Stealth Rocker on the team that also puts a dent in most things with it rather high attack stat.
Toxapex @ Eject Button
Ability: Regenerator
EVs: 252 HP/ 192 Def/ 64 SpD
Bold Nature
Moves:
Toxic Spikes
Haze
Recover
Infestation
It's Eject button is used for it to absorb any hit. Even it puts a dent in Toxapex, it'll always get the regenerator heal. It's also used so I won't let any other mon on my team to take a seriously hard hit. Toxic Spikes to wear grounded foes (that don't have magic guard and aren't part steel or poison or flying). The poison can also help it out with Mega Gyarados as it let's it stand a chance against Tangrowth (seriously why is Tangrowth such a problem to me sometimes). Haze to get rid of anyone trying to set up on it. Because those 63/52 offenses are definitely threatening to everything. Recover
Magearna @ Steelium Z
Ability: Soul Heart
EVs: 252 Spa/ 252 Spe/ 4 SpD
Timid Nature
Moves:
Shift Gear
Flash Cannon
Aura Sphere
Thunderbolt
This is meant to be the special wallbreaker. Or something like that. Shift gear to double it's trash speed. It allows it to compete with outspeed some fast threats such as Ash-Greninja, Tapu Koko, and others. Flash Cannon is used as it's main STAB attack. Pair it up with the Steelium Z, and you get something that let's you put a dent in things that aren't named Chansey. Aura Sphere put together with Flash Cannon gives you pretty good coverage on alot of things. I was going to run Focus Blast, but that 70% accuracy was so good, I didn't think I was worthy, so I put Aura Sphere instead. Thunderbolt also allows it too hit Toxapex, Zard Y, Gyarados, and non Lightning Rod Alolan Marowak's. Those were the mons that didn't take too much from either Flash Cannon or Aura Sphere. This set combined with Soul Heart, allows magearna to be an effective sweeper.
Zygarde @ Leftovers
Ability: Aura Break
EVs: 240 HP/ 116 Def/ 152 SpD
Careful Nature
Moves:
Dragon Dance
Protect
Thousand Arrows
Substitute
Zygarde is meant to be made as bulky as possible. It has dragon dance to let it boost it's speed and attack. It's meant too use DD behind the sub, so it'll evade the hit so it can boost it's stats at once. It also lets it boost it's speed so it can outspeed a lot of mons in the tier at +2 and even at +1. Protect to scout out for hits and to let it gain leftovers recovery. Thousand arrows to let it hit a lot of things pretty hard that aren't Grass, Bug, or Shedinja. It also lets it hit some mons that would otherwise wall it such as Lando, Skarmory, and Celesteela. This Zygarde can either be a sweeper and/or a staller. It can wear a lot of mons down very fast. +1 thousand arrows hits alot of the tier pretty hard as well.
Gengar @ Choice Scarf
Ability: Cursed Body
EVs: 252 Spa/ 252 Spe/ 4 SpD
Timid Nature
Moves:
Trick
Shadow Ball
Sludge Wave
Focus Blast
Gengar's Choice Scarf allows it to outspeed fast mons such as Mega Alakazam, Greninja and Tapu Koko without a Choice Scarf. and some other things. Shadow Ball is it's primary STAB move. It also takes advantage at the fact of how little of the tier resists ghost type attacks. Pair it up with Focus Blast, and you get perfect coverage of the tier. It also allows it to OHKO's Tyranitar, Bisharp, and other dark types on the switch. They all would otherwise threaten Gengar out. Sludge Wave let's it hit fairy and grass types for super effective damage. It also puts a dent in a couple of other things. Trick is used to punish bulky mons on the switch. Chansey and specially defensive Toxapex would wall this set. However, Trick can give them a Choice Scarf, which significantly hinders their use in the battle and allows Gengar to beat them. This set is used to capitalize on how Choice Scarf can allow it to beat many threats in the tier.
