Nicknames are up to you. Your team is not bad by any means but it could be improved with obvious fixes. If these fixes don't improve the team, I'll test it out myself and look into the problem in details.
Gyarados
- Crunch > Ice Fang
You need the Dark-type STAB as it is Gyarados' selling point as a Dragon Dance user after all: it makes him capable to take out bulky Psychic-types such as Jirachi and Slowbro. From this point on, I can recommend the second change quite easily:
Aerodactyl
- Change to Excadrill with this moveset:
Simon Drillmont (Excadrill) @ Leftovers
Ability: Mold Breaker
EVs: 252 HP / 4 Atk / 252 SpD
Careful Nature
- Earthquake
- Iron Head
- Rapid Spin
- Stealth Rock
Threatens Fairy- and Electric-types which Gyarados despises, provides Rapid Spin support and gives you Stealth Rock, all in one slot. Value! If it does not work, get rid of Stealth Rock and add another physical attack like Rock Slide, or Toxic to threaten bulky walls.
Why Aerodactyl doesn't work: He stacks the Electric-type weakness with Gyarados. Besides, it's counterproductive to invest in health and give a Focus Sash so your Aerodactyl set is very flawed.
Heatran
It's good. However, I generally prefer Volcanion as a Choice Specs user, but in this team it is fine to use Heatran as he doesn't stack the Electric weakness, and act as a lure to Ground-type attacks for your Gyarados to safely come in.
Thundurus-Therian
Gyarados is a lure to Electric-type attacks which Thundurus-Therian could take anytime so essentially Thundurus-Therian is in a great place in this team. However, your moveset is flawed because it is counterproductive to boost your Special Attack only to use your physical attack to get out of the matchup, essentially canceling both the Nasty Plot boost and the fact you're specially invested. I can understand why U-Turn as it does work on Ground-types but instead why not killing them?
Zeus (Thundurus-Therian) @ Leftovers
Ability: Volt Absorb
EVs: 252 SpA / 252 Spe / 4 SpD
Modest Nature
-Agility
-Thunderbolt
-Hidden Power Ice
-Focus Blast / Nasty Plot
By virtue of his amazing typing, Thundurus-Therian can force switches against a variety of threats, which give you the time to setup an Agility boost. The rest is straightforward: Click Thunderbolt unless you predict Steel- / Dragon- / Ground-type Pokemon coming. Nasty Plot is listed as an alternative to Focus Blast if the coverage is not needed, but don't get greedy and addicted to boosting.
The catch is, you should use Agility against faster teams and Nasty Plot against bulkier teams, allowing Thundurus-Therian to work as a stallbreaker, sweeper, Thunder Wave absorber all in one. Value!
Clefable
Standardize it.
Da Fat Cat (Clefable) (F) @ Leftovers
Ability: Magic Guard
EVs: 252 HP / 172 Def / 84 SpD
Calm Nature
IVs: 0 Atk
- Calm Mind
- Moonblast
- Thunder Wave
- Soft-Boiled
If you need a cleric, use Chansey instead. Blizzard is unneeded and inaccurate. Lum Berry is also useless since Clefable is meant to absorb Toxic and Will-O-Wisp incoming and take no damage from them. This set has studied EVs that allow it to dodge the 2HKO from specific attacks.
Amoonguss
Fine.
That's all.
