Garchomp doesn't have to use Leftovers (sweepers generally don't use Leftovers). I find that a Haban, Yache, or Roseli Berry is better to help it deal with its weaknesses.
Specs and Substitute don't mix. You're either gonna have to replace Gengar's Choice Specs with a Life Orb or Black Sludge, or replace Substitute with another special attack.
Goodra makes for an excellent special tank. Assault Vest is the better item here, since it allows Goodra to take special hits even better. And as a tank, it's not fond of the damage it takes from Life Orb use. There's also no need for both Draco Meteor and Dragon Pulse on it at the same time. Pick one, and replace the other with Thunderbolt or Sludge Bomb.
A Jolly nature on Cloyster makes Hydro Pump less effective. Either breed a new Cloyster with a Naive nature, or swap Hydro Pump with Razor Shell.
Your team is offensively inclined, but your Aggron doesn't fit that. Its Defense, especially when mega evolved, is absolutely fantastic, so Iron Defense isn't needed. Steel now has use offensively, but that doesn't warrant 2 Steel attacks on Aggron. So pick between Iron Head and Metal Burst (I prefer Iron Head, but its up to you). If you want an offensive (Mega) Aggron, it'd look more like this:
-Mega Aggron w/Aggronite
Nature: Adamant/Jolly
Ability: Sturdy----->Filter
EVs: 252 Attack, 252 Speed, 4 HP
Moves: Iron Head, Automotize, Earthquake, Brick Break/Stone Edge
I see Mega Aggron as a more defensive Pokemon, but that could work.
or
-Aggron w/Focus Sash or Leftovers or Air Balloon
Nature: Adamant
Ability: Rock Head
EVs: 252 Attack, 252 Speed, 4 Sp.Defense
Moves: Automotize, Head Smash, Earthquake, Iron Head
or
-Aggron w/Choice Band
Nature: Jolly
Ability: Rock Head
EVs: 252 Attack, 252 HP, 4 Sp.Defense
Moves: Head Smash, Earthquake, Iron Head, Brick Break
Nah ンン
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