(I assumed the format was Gen8 OU Singles because you didn't mention anything)
I had a couple of suggestions for each member:
It seems like you're going for a bulky Corviknight. If so, you should replace Brave Bird with Drill Peck. Losing the HP isn't worth it. But the other moves work well, especially Iron Defense and Body Press.
Another thing people in the comments have picked up is that the 4 EVs in SpA. Not sure if this is a mistake, but all your moves are physical, so go for 4 EVs in Attack instead.
And the Impish Nature- people tend to prefer Careful, but I recommend you play around with both and see which one works best.
Corviknight @ Leftovers
Ability: Mirror Armor
EVs: 252 Def / 4 Atk / 252 SpD
- Drill Peck
- Body Press
- Iron Defense
U-turn was a good option (after a bit more research), but you could also try Swords Dance. It would work well with a Focus Sash
Leech Life is genius, because it has the same power as X-Scissor, and also an additional bonus of healing. Protect and Acrobatics are nice too because they can stall and utilize the loss of an item respectively.
I would say a Jolly Nature because Speed is the top priority on a Ninjask.
Ninjask @ Focus Sash
Ability: Speed Boost
EVs: 252 Atk / 4 SpD / 252 Spe
- U-turn/Swords Dance
- Leech Life
Hippowdon is pretty well done, but I'd recommend Slack Off over Rest, because you'll stay awake and you already have a Lum Berry.
Toxic was a good choice, and an Impish Nature is nice too.
You probably have a good reason for Sand Attack, but if the opponent switches out it goes to waste. Try Stealth Rock instead. It can be slightly predictable, but it is a great move, especially on Hippowdon.
Hippowdon @ Lum Berry
Ability: Sand Stream
EVs: 252 HP / 4 Atk / 252 Def
IVs: 0 Atk
- Rest/Slack Off
- Body Press
An Adamant Nature would be superior to a Brave one. There's no reason to lower your Speed when you can lower Special Attack.
I saw that 4 EVs were left- you could put that in Defense.
Other than that, I thought Scizor was set out quite well! Your moveset is clever, using Bullet Punch to get boosts from Technician, and priority.
Scizor @ Focus Sash
EVs: 252 Atk / 252 SpD /4 Def
- Bullet Punch
- Brick Break
- Swords Dance
Garchomp has two STAB moves, so either Outrage or Dragon Claw's gonna have to go. Earthquake and Swords Dance are great moves but predictable. Not a problem, just keep that in mind.
The remaining slot should be a coverage move, either Poison Jab for Fairies or Fire Fang for its 4x weakness to Ice (Garchomp may be weaker to Ice, but Fairy moves are more common, so keep that in consideration).
To take more hits use the spare 4 EVs for HP.
Haven't seen a Garchomp with a Rocky Helmet yet, but I don't think it'll turn out badly.
Garchomp @ Rocky Helmet
Ability: Rough Skin
EVs: 252 Atk / 252 Spe /4 HP
- Dragon Claw/Outrage
- Fire Fang/Poison Jab
- Swords Dance
A Jolly or Adamant Nature would be better than Impish, as Sandslash is more dependent on its offense. Icicle Crash is probably better than Avalanche, because at Sandslash's Speed it's not that rare to find a Pokémon slower than it. A Jolly Nature also becomes more viable without it.
Metal Burst could work well, but it can be a bit of a gamble, and you shouldn't risk it. Iron Head would be a better choice.
On the contrary, Super Fang was a good selection, and so was Focus Sash, but a Life Orb can be considered too if going for a hyper offensive Sandslash.
Sandslash-Alola @ Focus Sash/Life Orb
Ability: Slush Rush
EVs: 252 Atk / 252 Def
- Super Fang
- Iron Head
- Icicle Crash
(That was longer than I wanted it to be. My apologies for that and also if I sounded harsh.)
And that's it! Remember that this is only my opinion. You don't have to listen to any of them. In fact, I encourage you to try your original team out and see which parts need polishing.
Hope I helped! :)
(Thanks to IncineroarforGods for telling me that Baton Pass was banned)
Side note: I am no expert at Pokémon battles. Don't come and kill me because I was incorrect. Please.