Garchomp
Level:70
Moveset
Draco Meteor
Steath Rock
Fire Fang
Dragon Tail
Beside the Draco Meteor, I believe a Steel-Type attack instead of Fire Fang would work better. It will get rid of Ice- and Fairy-Types, as they are both weak to Steel. Otherwise, this is good. I would prefer Iron Head over Iron Tail, mainly because Iron Tail is less accurate and Iron Head has a 30% chance to flinch. You can put a King's Rock on Garchomp to make the flinch chance even worse.
Aegislash (Blade Forme)
Level: 67
Moveset
Fury Cutter
Rain Dance
Sacred Sword
Brick Break
Why Rain Dance? No other Pokemon except for Greninja is going to benefit from this. Brick Break is handy to get rid of screens, but I find Defog more useful for getting rid of not only screens, but hazards on the field. I would suggest swapping out Rain Dance for Swords Dance, so you may have stronger Attack. And King's Shield is like a form-changing Protect, so invest that somewhere if you'd like. Without it, Aegislash won't survive long. You'll need a Pokemon to have a Water-Type attack to switch Aegislash out of battle from a Fire- or Ground-Type Pokemon. Fury Cutter is only good on Dark-Types and it isn't all that great to be honest. X-Scissor is more reliable. But you'll need Shadow Claw, so maybe replace Fury Cutter with either Shadow Claw or X-Scissor. If you want to get rid of Sacred Sword for a better Steel-Type STAB, go for Iron Head. Since Greninja has got the Fire- and Ground-Type weakness of Aegislash covered, you are good.
Cornibus
Level: 65
Moveset
Charge
U-Turn
Giga Impact
Brick Break
Who is Cornibus? And why have Charge when you have no Electric-Type moves to benefit from this? Unless you just want a Special Defense raising move, but there are better moves out there for that. I have no idea if Cornibus is what you meant, but I can't reliably help you on this one without knowing its type or possible abilities.
Snorlax
Level:69
High Horsepower
Heavy Slam
Hammer Arm
Belch
Belch is an interesting move and I do like seeing more of it, but Snorlax has to have Recycle if you want to use it better, so that you can keep using Belch. Other than that, these are all pretty good moves except you can't use Belch reliably without Recycle, so Return might work better than Belch/Recycle. I would suggest Crunch or Darkest Lariat (yes, it can learn this via TR93) to replace Hammer Arm, so you may actually hit Ghost-Types, especially those with Levitate.
Greninja
Level: 70
Water Shuriken
Hydro Pump
Close Combat
Acrobatics
Greninja is weak to Fighting, Bug, Grass, Electric, and Fairy, if you don't plan on having Protean on this thing. Heavy Slam and the Iron Heads can protect Greninja from any Fairy-Type, Acrobatics is good against Fighting, Bug, and Grass, and you have High Horsepower to protect Greninja from Electric-Type attacks. Sadly, you have to choose between Water Shuriken or Hydro Pump. Because there is another move you'll need. If you want to set up hazards against your opponent, have it learn Spikes or Toxic Spikes in place of one of these two. If you don't want that, but want coverage instead, go for Ice Beam for extra coverage.
Also, just realized Greninja is Special and Physically oriented. I'm assuming you want a mixed set, so there you go. You don't exactly have a Pokemon as a wall, and you're gonna need one. Snorlax is a wonderful wall and a physical sweeper. If you have any intentions on changing that up a little bit, I can help.
These are mostly just suggestions, as Pokemon battling is really nuanced. People have actually tried out Leon's team in Sword and Shield and found that he can literally bust through most of the gym leaders and reach the top, unlike some other Gym Leaders or the rival's Pokemon, so selection of Pokemon and their roles in battle are key.
