I have a few comments about this team. On all Pokemon you're running 4 HP EVs, while they are usually set in one of the Defense stats in their standard sets. However, it's really up to you, since it doesn't make that much of a difference.
Pheromosa
This is fine. I would run 4 SpD EVs and a Focus Sash, but really, this thing is just a glass cannon.
Dragapult
Personally, I would run a Physical set since its Attack stats are 120 to 100, and its Signature Move, Dragon Darts is better than Draco Meteor. Smogon says Choice Specs is better, so I guess it's okay. I think U-turn should be somewhere on the set, since Dragaput is really fast and with U-turn it can pivot out against Dark-types that threaten it.
Magearna
Lots of Choice items here, eh? I think by 30 Atk IVs you mean 0. I wouldn't run Steel Beam unless you have a reliable Wish supporter like Chansey or Blissey, as Magearna lacks recovery. Replace it with Flash Cannon since there are no drawbacks to it. I think Aura Sphere, which is, I'm assuming, to counter Ferrothorn, could be replaced with Trick to break stall (like Ferrothorn).
Cinderace
Since this is a Court Change set, I would definitely run Heavy Duty Boots over Life Orb. I would also run Sucker Punch instead of Gunk Shot because it's priority and can be quite damaging against Dragapult and Spectrier.
Slowbro
I don't really understand Heavy Duty Boots on this one. Sure, Smogon may say it can help, but it's not weak to Stealth Rock, and Cinderace will get rid of any other hazards that affect it. I would also run Psyshock over Future Sight, but it should be fine. I would say get a Calm Nature instead of Relaxed. Only run Minimum Speed if it's a Trick Room team.
Garchomp
Life Orb is probably better than Leftovers here, since Garchomp appreciates the extra power boost to Scale Shot, which has a minimum BP of only 50. This also is your second Dragon type, but since you have a Fairy / Steel I think you can keep both.
So yeah, these are my recommendations for this OU team. Note that I am not very good at OU, so any comments on this are appreciated. Hope I helped!
