Hey there! Mega Gyarados balance is a pretty cool concept and I like some of the defensive core going on. However, the team is very weak to Mega Manectric and Kyurem-Black. If Mega Manectric is able to switch in, there's nothing on this team that can take it on 1v1. Similarly, nothing on this team wants to switch into Kyurem-Black's attacks, and Garchomp can't even 1v1 it because it only has 4 Speed EVs which means that it doesn't outspeed Kyurem-Black. In addition, Mega Venusaur completely walls your team once Skarmory's gone. You don't have any hazard setters on this team, which is something that just about every team should try to have. Speaking of hazards, you have no Defog or Rapid Spin user, which is not a good thing on a balance team since hazards can wear down the defensive core that balance teams have, allowing for set-up sweepers to be able to sweep the team given the right opportunity. The last major problem I see is that all of your sets are off in some way.
To have a switch-in to Mega Manectric and a more reliable pokemon to check Mega Venusaur, I suggest replacing Chandelure with Heatran.
Heatran @ Leftovers
Ability: Flash Fire
EVs: 252 HP / 188 SpD / 68 Spe
Calm Nature
- Lava Plume
- Toxic
- Taunt
- Stealth Rock
Chandelure is not really viable in OU because its poor defensive stats and speed means that it gets outclassed by different fire type pokemon. Heatran fits much better in this team because it counters Mega Venusaur and checks Mega Manectric, which nothing on your team could really do. Additionally, adding Heatran gives you a hazard setter and a Taunt user to help break through stall teams. Talonflame could be a problem once Garchomp goes down, and Heatran gives the team a counter to it which helps the matchup versus Talonflame.
This team has some redundancy type-wise. Having Azumarill and Mega Gyarados on the same team only stacks weaknesses to electric types, since Azumarill isn't dealing with anything that another fairy type pokemon can't. Because of this and because this pokemon helps with your Kyurem-Black weakness, I suggest replacing Azumarill with Clefable.
Clefable @ Leftovers
Ability: Magic Guard
EVs: 252 HP / 172 Def / 84 SpD
Calm Nature
- Calm Mind
- Moonblast
- Soft-Boiled
- Thunder Wave
Clefable can switch into any of Kyurem-Black's moves, bar the rare Iron Head. Clefable can also switch into any of Mega Manectric's moves, helping the matchup versus it even more. Having Clefable on the team gives you an extra win condition against stall teams as well as balance teams in the event of Mega Gyarados going down. Thunder Wave provides great utility on a slower team like this, paralyzing fast attackers.
To start with the set changes, Garchomp should be running a Choice Scarf set to provide some speed and power for this team.
Garchomp @ Choice Scarf
Ability: Rough Skin
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Earthquake
- Outrage
- Stone Edge
- Dragon Claw
TankChomp is the most common set that Garchomp runs, so having a Choice Scarf set can provide an element of surprise since this set isn't very common in the metagame right now. Although it's uncommon, it's the best set for this team since power and speed are needed, a hazard setter already exists, and there are already 2 win conditions so a third one isn't needed (if another win condition was needed, I would have likely suggested Swords Dance Garchomp).
Skarmory is used as a defensive pokemon, so running a Swords Dance set isn't a viable option for this pokemon. Instead, I would replace the current set with the Defog set.
Skarmory @ Leftovers
Ability: Sturdy
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Roost
- Defog
- Brave Bird
- Whirlwind
Using Defog Skarmory gives you a hazard remover, which as I've mentioned earlier, is an important thing to have on a balance team such as this.
Amoonguss' set should be changed to the following set:
Amoonguss @ Black Sludge
Ability: Regenerator
EVs: 248 HP / 168 Def / 92 SpD
Bold Nature
- Spore
- Giga Drain
- Clear Smog
- Hidden Power Fire
92 Special Defense EVs ensure that Amoonguss avoids the 2HKO from Specs Keldeo's Icy Wind, and the rest of the EVs are put into HP and Defense to maximize its bulk and to switch into hazards the maximum amount of times. Black Sludge is a slightly better item than Leftovers for poison types because if Black Sludge is tricked onto another pokemon that's not a poison type, it loses 1/8th of its HP every turn. While Clear Smog is weaker than Sludge Bomb, it should still be used over Sludge Bomb because of its ability to remove stat boosts and because Amoonguss isn't used as an offensive pokemon to begin with. Protect isn't really necessary on Amoonguss, while Hidden Power Fire can hit Ferrothorn and Mega Scizor, the former of which can be annoying once Heatran goes down.
Finally, Mega Gyarados' set should be changed to the following:
Gyarados @ Gyaradosite
Ability: Intimidate
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Dragon Dance
- Crunch
- Waterfall
- Ice Fang
Mega Gyarados should be running maximum Attack and Speed EVs to outspeed certain pokemon both unboosted and at +1, and to maximize its offensive capabilities. Dragon Tail isn't a useful move on Mega Gyarados, while Earthquake isn't necessary for this team because of coverage existing for the pokemon that are hit by Earthquake. Crunch is a powerful STAB move that shouldn't be passed up, and Ice Fang hits Grass and Dragon type pokemon for a lot of damage.
Summary of Changes:
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