Okay, here we go. First of all, after watching you use this team, I can see that you are a fairly new player. (1100 elo) A lot of the advice I am going to give is mostly applicable in the higher up ladder (About 1400 is where things get serious), so it may not be immediately useful.
But first, just some general advise for starting off. You should really memorize the type matchup chart, or just have it open in a browser tab when you play (Drain Punching a Dragonite...) Also, switching out is very important, don't keep your Ferrothorn in when you know the opponent has Fire Blast, switch to Diance or Ceruledge. Also, try to predict your opponent's moves, and get some sort of knowledge of what's common and what's not. (For example, opponent had Dragonite, Iron Valliant used Spirit Break, it did abut 40%, you used Drain Punch, presumably to chip for the next Spirit Break and recover some HP. However, pretty much all Singles Dragonites run Multiscale, meaning that a second Spirit Break would have KOed.) Fizz's new website Gatrstats is actually very helpful with this.
Me rambling aside, let's get to the team.
In total, your team has resistances to all but Ground and Ghost. You have two Ghost weaknesses, which is okay, because Ghost isn't a very common coverage type, so you should be able to see Ghost type moves coming. Ground however... half your team is weak to Ground, and Ground is a VERY common type. Nearly every Physical sweeper gets Earthquake. The best thing you can do is to put a Flying type on your team, or put an Air Balloon on Gholdengo. Also, your Tera types are meaningless, because Tera is Banned in Natdex.
Now on to each of the team members:
Mega Diancie
I was never a fan of Mixed Mega Diancie. Full Special is just so much better. Here is the set that I use:
Diancie-Mega @ Diancite
Ability: Clear Body
EVs: 252 HP / 252 Def / 4 Spe OR 252 HP / 252 SpA / 4 Spe
Modest Nature
IVs: 0 Atk
- Calm Mind/Power Gem
- Earth Power
- Draining Kiss/Moonblast
- Rock Polish
You can either choose to go with Calm Mind, and use the First EV spread, and the first moves, or you can use the Second EV spread and swap Calm Mind for Power Gem and Draining Kiss for Moonblast. Whichever way you choose, remember to set up Rock Polish (And Calm Mind if you choose it) while in non-Mega form, because non mega has better defenses. After one Rock Polish, you're ready to go.
Gholdengo
You should replace Assault Vest with Air Balloon, and swap Flash Cannon for Nasty Plot. With a Nasty Plot boost, Gholdengo can afford to use Make It Rain without worrying about the SpA Drop. Air Balloon is better for your team, and Assault Vest is better for a bulky EV spread.
Ferrothorn
Looks ok... You should replace Toxic with Power Whip and change the Nature to Impish. This will help Ferrothon be less passive, and give you some Grass coverage.
Ceruledge
Shell Bell is a garbage item that is only used by trolling Level 1 Sturdy Pokémon. 10% of Damage done is barley anything, and Leftovers heal more, and do it consistently. Dark has worse coverage than Ghost, and doesn't get STAB. (In case you didn't know, when a Pokémon's move is the same type as it, it gets a 50% power boost.) Here is a better set:
Ceruledge @ Weakness Policy
Ability: Weak Armor
EVs: 4 HP / 252 Atk / 252 Spe
Adamant Nature
- Bitter Blade
- Endure
- Close Combat/Shadow Sneak
- Poltergeist
This is a pretty standard Weakness Policy Ceruledge set. The idea here is to have Ceruledge be fighting a Pokemon with Physical Super Effective moves against it, use Endure to survive it, Weak Armor Activates, boosting your speed, and Weakness Policy activates, boosting your Attack. You then proceed to sweep. This looks like a niche situation, but Earthquake is more common then you think. You do get destroyed by priority, so either avoid fighting Priority users, or just use your own in Shadow Sneak.
*Important Note: ONLY use Poltergeist on Opponents with Rocky Helmet or Rough Skin. I have lost too many battles by using Bitter Blade. The damage from Rocky Helmet/Rough Skin happens BEFORE you get HP back.*
Iron Valiant
EV Spread is fine... but the moves need fixing. Use Close Combat over Drain Punch, Iron Valliant isn't tanky enough to use Drain Punch well. Spirit Break is fine. Throat Chop though... that needs to be replaced. Replace it with Shadow Claw, Knock Off, Psycho Cut, Ice/Fire/Thunder Punch, ANYTHING ELSE. Also use Swords Dance over Protect. This ain't doubles, and a power boost is just what Iron Valliant needs to sweep.
Zamazenta
It's alright. Use Leftovers over Rocky Helmet, recovery is better than chip in this case. You could keep protect, for Leftovers Healing, but you could also use Roar to get out of bad matchups, AKA, Ghost types. Also consider: Crunch. (For dealing with those stupid Ghost types) Substitute is another option too.
IronPress Zamazenta is fine, but let me suggest: Choice Band Zamazenta.
Who let him in OU? (Zamazenta) @ Choice Band
Ability: Dauntless Shield
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Close Combat
- Crunch
- Iron Head
- Wild Charge
You could also replace one of the coverage moves with a different one, like Psychic Fangs, Play Rough, or whatever.
Whichever one you choose, they're both great.
Tl;DR
JK there is no tl;dr. Read the answer I worked hard on.
In conclusion...
Your team has a promising start. With these changes, I'm sure you will do at least a little better. Remember, practice makes perfect. I look forward to battling you eventually...
Hope I helped.
