Am I using a good competitive team in ORAS?

This is the party I use for competitive single battles in ORAS, I was hoping somebody could evaluate it and tell me where I can improve. Thanks!

Garchomp @ Rocky helmet
Ability: Rough skin
EVs: 252 Atk/ 4 SpD/ 252 Speed
Nature: Adamant
-Stealth rock
-Earthquake
-Dragon tail
-Fire blast

I always lead with Garchomp and set up Stealth rock immediately. I then use Dragon tail to keep switching out the opposing Pokémon and chip away at every member of the opponent's party due to stealth rock damage. Fire blast is a good supereffective move against steel types and Earthquake provides for a powerful STAB. The Rocky helmet combined with its Rough skin ability takes 30% of any physical attacker's HP.



Flygon @ Leftovers
Ability: Levitate
EVs: 252 Atk/ 4 Def/ 252 Speed
Nature: Adamant
-Earthquake
-Outrage
-U-turn
-Dragon claw

Flygon is my go-to Pokémon for taking out enemies in one or two hits. Dragon claw and Earthquake are decent STABs with no negative effects. If the opponent sends out an Ice or Fairy type Pokémon, I always use U-turn to avoid Flygon getting KO'd and deal some damage at the same time. If the opponent sends out a Dragon type, Flygon can KO it in one hit due to its high speed and Dragon claw. Leftovers slowly heal Flygon over time, giving it a slight stamina boost and Outrage is an extremely powerful STAB, but I only use it when the battle is almost over or Flygon is about to faint.



Aggron @ Aggronite
Ability: Sturdy
EVs: 252 HP/ 16 Def/ 240 SpD
Nature: Impish
-Rest
-Iron head
-Heavy slam
-Snore

Aggron is my TANK! It has a monstrous amount of HP and an insanely high Special Defense as well, so all in all, he takes a long time to KO. Iron head is an amazing STAB, especially due to its high flinch ratio. Since Aggron is the Pokémon that I Mega Evolve, Heavy slam is extremely powerful due to Mega Aggron's insane weight of 870.8 lbs, KO'ing almost any Pokémon. In the unlikely scenario that the opponent gets Aggron into low HP, Aggron can use Rest and keep using Snore until it wakes up.



Dusknoir @ Eject button
Ability: Pressure
EVs: 252 HP/ 252 Atk/ 4 Def
Nature: Adamant
-Shadow sneak
-Earthquake
-Ice punch
-Trick

Dusknoir is not a 100% troll, but it can definitely be a pain to eliminate. I always use Trick first and give the opponent the Eject button. Shadow sneak makes up for Dusknoir's terrible speed and even though it is not a very powerful move, it gets a STAB boost. Ice punch deals massive damage to Flying, Ground, Grass, and Dragon type Pokémon while Earthquake is just a regular, semi-powerful move.



Toxicroak (SHINY!) @ Life orb
Ability: Dry skin
EVs: 252 Atk/ 4 SpD/ 252 Speed
Nature: Jolly
-Toxic spikes
-Gunk shot
-Drain punch
-Sucker punch

If I do not need to send out Flygon, I send out Toxicroak after Garchomp. He is extremely fast and will almost always be able to set up Toxic spikes before the opponent can make a move. Sucker punch is a good priority move and if it can KO the opposing Pokémon, Toxicroak can avoid taking any damage that turn. Gunk shot is a powerful stab, but I may decide to switch it with Venoshock since every opposing Pokémon will be poisoned due to the Toxic spikes. Its Jolly nature boosts its Speed, ensuring that it can set up its Toxic spikes before the opponent can attack and its Dry Skin ability gives it a huge advantage over Water types. Drain punch's only purpose, as well as being a semi-decent STAB, is to recover HP that is lost due to the Life Orb. Besides, it is SHINYYYYYY!



Gallade @ Focus band
Ability: Steadfast
EVs: 252 Atk/ 4 SpD/ 252 Speed
Nature: Adamant
-Close combat
-Psycho cut
-Swords dance
-Drain punch

Gallade is my sweeper. If I get the chance, I will use Swords dance once or even twice, allowing Psycho cut and Drain punch to KO almost any Pokémon. Even without Swords dance, Close combat is a reliable STAB that will OHKO most Pokémon. The fact that Close combat lowers Defense and Special Defense combined with Gallade's poor Defensive stats means that almost every attack that hits Gallade will OHKO it. For this reason, I gave Gallade the Focus band. Gallade will always attack first due to its incredible speed, meaning that if the opposing Pokémon OHKO's Gallade, it can use Drain punch and restore most of its HP.

Usually when you run a Garchomp with Rocky Helmet and Dragon Tail you want to run an EV spread invests in bulk so it can actually make use of the helmet+Rough Skin and Dragon Tail has negative priority. Smorgen recommends a spread of 240 HP/176 Def/92 Speed

There's not really any reason to use Flygon tbh. It's rather redundant when you have Garchomp, who is straight up better than Flygon due to better stats and movepool. You're also just adding another 4x Ice weakness, a Dragon weakness, and a Fairy weakness to the team. And with an Adamant nature you're guaranteed to be outsped by enemy Salamences, +Speed Hydreigons, and offensive Garchomps.

Putting both Iron Head and Heavy Slam on Mega Aggron makes it a lot easire for shit to come in and wall it. You need to replace one of them with a coverage move. Snore is also weak and it'd be better to put Sleep Talk in its place to get you a shot at having it actually doing something while asleep (ie using Steel STAB or coverage move).

Gallade is too slow to sweep. It can wallbreak (though Mega Gallade does that better), but again, it's stacking weaknesses on your team.

I'd also swap Dusknoir with Sableye. Dusknoir's not really doing much for the team after it uses Trick (also Trick+Eject Button is a one time thing, Trick+Choice item can cripple a 'mon for the whole match). Sableye also gets Trick but has Prankster, Recover, and a good support movepool to help your team out much more.

As for replacements for Flygon and Gallade.....there's always like Azumarill, Serperior, Gyarados, idk

Interesting team. Good call on covering up Garchomp and Flygons double weaknesses to ice. Plus Aggron can cover fairies. I wouldn't put FIre Blast on Garchomp personally just because of the poor accuracy it has but if it works then power to you. Can I battle you sometime and test my luck on this team

As Nah already mentioned, Dragon Tail isn't good on offensive Garchomp because the negative priority messes offensive Garchomp hard, and is better off on a defensive set. If you still want to use Garchomp for offensive purposes, here's a better take:
-Stealth Rock
-Outrage/Dragon Claw
-Earthquake
-Fire Blast/Swords Dance
Nature: Naive/Jolly
EVs: 252 Atk/4 SAtk/252 Spe
Item: Lum Berry/Rocky Helmet/Focus Sash

You shouldn't even be using Flygon in standard play; as already mentioned, it's severely outclassed by Garchomp. Snore is a bad attack on Mega Aggron because it's a weak Normal attack that doesn't add on to its coverage. If you want a better Steel Pokemon, use (Mega) Scizor, as it can also use U-turn for scouting purposes if needed:
-Swords Dance
-U-turn/Superpower/Knock Off
-Bullet Punch
-Roost
Nature: Impish
EVs: 248 HP/44 Atk/16 Def/200 SDef
Item: Scizorite
Ability: Technician

or
-Swords Dance
-Bug Bite
-Bullet Punch
-Superpower
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Scizorite/Life Orb
Ability: Technician

or
-U-turn
-Bullet Punch
-Superpower
-Pursuit/Knock Off
Nature: Adamant
EVs: 248 HP/252 Atk/8 SDef
Item: Choice Band
Ability: Technician

Dusknoir is one of the worst Ghost Pokemon, mainly because it lacks a good Ghost STAB. Gengar and Mega Sableye (only if you're not using Mega Scizor, as you can only Mega Evolve 1 Pokemon per battle) are pretty much the only viable Ghost Pokemon out there, IMO. The former has superior offensive presence than Dusknoir, while the latter has a good support move pool to take advantage of its good defenses:

Gengar:
-Shadow Ball
-Sludge Wave/Sludge Bomb
-Focus Blast
-Taunt/Icy Wind/Destiny Bond/Substitute
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Life Orb/Focus Sash/Black Sludge

or
-Hex
-Taunt
-Will-o-Wisp
-Sludge Wave/Sludge Bomb/Substitute
Nature: Timid
EVs: 8 HP/248 SAtk/252 Spe
Item: Black Sludge

Sableye:
-Knock Off
-Will-o-Wisp
-Foul Play/Toxic
-Recover
Nature: Careful
EVs: 252 HP/112 Def/144 SDef
Item: Sablenite
Ability: Prankster

or
-Calm Mind
-Dark Pulse/Shadow Ball
-Will-o-Wisp
-Recover
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Sablenite
Ability: Prankster

Toxicroak can't learn Toxic Spikes. While Toxicroak is viable in its own right because of its ability to easily handle most Water Pokemon thanks to Dry Skin, I feel that the alternatives Nah suggested are better. However, the Scizor suggestion leaves you open with an extra slot, so spend that slot on Wash Rotom, as its Volt Switch pairs well with Scizor' U-turn should you use that:

Serperior:
-Leaf Storm
-Dragon Pulse
-Hidden Power (Fire)
-Giga Drain/Substitute/Taunt/Glare
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb/Leftovers
Ability: Contrary

Azumarill:
-Waterfall
-Aqua Jet
-Play Rough
-Superpower/Knock Off
Nature: Adamant
EVs: 172 HP/252 Atk/84 Spe (Choice Band) or 240 HP/252 Atk/16 SDef (Assault Vest)
Item: Choice Band/Assault Vest
Ability: Huge Power

or
-Belly Drum
-Aqua Jet
-Play Rough
-Knock Off
Nature: Adamant
EVs: 92 HP/252 Atk/164 Spe
Item: Sitrus Berry
Ability: Huge Power

Wash Rotom:
-Volt Switch
-Hydro Pump
-Will-o-Wisp
-Pain Split/Rest
Nature: Bold
EVs: 248 HP/252 Def/8 Spe
Item: Leftovers/Chesto Berry

Regular Gallade is outclassed by Mega Gallade because the latter is stronger, faster, and is physically bulky enough not to be KOed by a decently-powered attack. If you didn't opt for Mega Sableye or Mega Scizor, this is what Gallade should be running:
-Swords Dance
-Zen Headbutt
-Close Combat/Drain Punch
-Knock Off/Shadow Sneak/Ice Punch
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Galladite

There aren't too many viable non-Mega Evolved Psychics out there, though.