So I'm noticing a fair number of problems with the team. No problem we all start somewhere. To start, Skarmory is a very good hazard setter. However your EVs are a little off.
Skarmory @ Leftovers
Trait: Sturdy
EVs: 252 HP / 232 Def / 24 Spd
Impish Nature
- Whirlwind
- Roost
- Brave Bird
- Spikes
So firstly, I changed the EVs to be a fully Defensive pokemon for you. I think Defensive set will do just fine. I'll explain more fully when I show you your next poke, who is replacing Ninetales.
Ninetales
Thumbs up for using Flash Fire Tales, I used it and I liked it xD However with Defensive Skarm on the team there is someone who can abuse Flash Fire even more than Ninetales, Heatran
We will be making Tran Specially Defensive to take special hits directed at Skarm. The beauty of these two steel types is that neither of them share a weakness with the other. Now they don't cover each other's weakness with resisting, but it creates a pseudo core.
Heatran @ Leftovers
Trait: Flash Fire
EVs: 252 HP / 200 SDef / 52 SAtk
Calm Nature
- Stealth Rock
- Roar
- Lavaplume
- Hidden Power [Ice]/Earth Power
Lavaplume is main STAB attacking move and has a good chance to burn. SRocks is to aid Skarm in hazard setting. You can go for Roar for the same reason you use Whirlwind on Skarm. That way both your wallers have a switching move. HP Ice hits dragons like Garchomp on the switch, but Earth Power hits other Heatrans and water/fire types who will gladly switch in. EVs provide SDef bulk with a few more EVs in SAtk to hit just a bit harder.
That is your two part pseudo core to start. Let's travel on to the rest of the pokes.
Toxicroak
I'm going to replace Toxicroak as he doesn't add much to the team at this point. His replacement will be a bulky special attacker who will synergize perfectly with your pseudo core SkarmTran.
Gastrodon @ Leftovers
Trait: Storm Drain
EVs: 252 HP / 200 SAtk / 52 Def
Modest Nature
- Surf
- Ice Beam
- Earth Power
- Recover
Switch Gastro into any water move you expect your opponent to attack Skarm or Tran with to get a boost to your SAtk then begin sweeping. The set provides you a way to hit every pokemon at least neutrally while Recover is there to annoy your opponent to no end with healing.
Haxorus
I think this is a great poke for the team as his weaknesses are covered by Skarm and Tran. I'm going to change your set a little to better abuse his ability.
Haxorus @ Lum Berry
Trait: Mold Breaker
EVs: 220 Spd / 252 Atk / 36 HP
Adamant Nature
- Outrage
- Dragon Dance
- Earthquake
- Superpower
I give him Lum Berry for two reasons.
A) So he can set up without being paralyzed or burned
B) So that if you set up without using Lum then you can Outrage sweep, get confused, cure confusion, and sweep some more.
My advice is, don't use Outrage when your opponent has Skarm or Forretress still alive. Those two love switching into Outrages and living them with ease. Tran can also be a problem but he is more often specially defensive as opposed to defensive, so you can run that risk if you like. Earthquake is to allow you to abuse your ability and hit any levitating pokemon, chiefly among them Rotom-W. Superpower finishes the set off.
So now on your team you have,
1 - Defensive Pokemon
1 - Specially Defensive Pokemon
1 - Special Attacker
1 - Physical Attacker
So we can either add one pokemon to the last two slots to create a
team with an offensive bias, or we can add two pokemon to the first
two slots to create a defensive bias, or we can throw in
another wall and a mixed attacker for a mixed bias. Of these
options a lot will depend on your playstyle, I personally would go for
mixed attacker + a wall. So I'll suggest some pokemon to fit that.
I'll also suggest two more attacking pokemon for an offensive bias. It
requires less prediction and thinking to maintain an offensive bias
instead of a defensive bias.
Mixed Bias
+1 Attacker
+1 Defender
Infernape can run one of the most balanced mixed attacking sets in the metagame. A mixed set I have been using recently has had wonderful success, gaining many KOs and allowing me to surprise the foe.
Infernape @ Life Orb
Trait: Iron Fist
EVs: 252 Spd / 100 SAtk / 156 Atk
Naive Nature
- Close Combat
- ThunderPunch
- Hidden Power [Ice]
- Fire Blast
Two STAB moves and two for coverage. The one thing I absolutely love about Infernape is that it can't be burned, which is awesome. Life Orb boosts your moves power and ensures some KOs as the EVs have been spread out. Iron Fist boosts ThunderPunch and Close Combat, Fire Blast is just ultimate power while HP Ice hits all of those annoying dragon types and Gliscor/Lando.
Another set you can try which was mention by my good friend Blobyolo is this,
Infernape @ Expert Belt
Trait: Iron Fist
EVs: 252 Atk / 252 Spd / 4 SAtk
Hasty Nature
- Overheat
- Close Combat
- Mach Punch/U-Turn
- Hidden Power [Ice]
Both sets are good. The second is the classic version and works very well.
A word from Blobby:
> Overheat is nice as it also helps bluff the scarf if you are using a hit and run set :P. It also hits harder meaning there is no use for the SATK evs + allows you to fully invest in ATK. This allows you to hit stuff like Politoed on the switch in for about 75%
Now with the introduction of Infernape there is a little too much water weakness. To balance this I'm going to add in a pokemon that synergizes perfectly with Heatran and gives you a four part core with Skarmory, Tran, Gastro, and
Celebi @ Leftovers
Trait: Natural Cure
EVs: 252 HP / 76 Def / 180 SDef
Impish Nature
- U-Turn
- Thunder Wave
- Leech Seed
- Giga Drain/Substitute/Protect/Other
TWave and Leech Seed are all you need to cause a ragequit. But sadly there is a chance someone may dare to taunt you. In that case you can U-Turn out of them or Giga Drain. Substitute or protect is to allow for stalling, I personally would go for Sub > Protect as with Leech Seed you will be gaining you HP back every turn, and it will be hell to take down. With Heatran, Celebi forms a CeleTran core which is arguably one of the best ones. With Skarm and Gastro thrown in you have several switching possibilities that will constantly keep the foe on their toes.
Offensive Bias
+1 Special Attacker
+1 Physical Attacker
To fill the role of special attacker I will be using someone who can also take some fighting hits for your two steel types. Gengar.
I think the best Gengar to use here would be full out offense with a LO Gengar. I recently suggested one to another person so that's the set I'll give to you.
Gengar @ Life Orb
Trait: Levitate
EVs: 252 SAtk / 252 Spd / 4 HP
Timid Nature
- Shadow Ball
- Focus Blast
- Thunderbolt
- Pain Split/Giga Drain
When holding LO you want to make sure that your total HP cannot be divided by 10 to reduce the recoil. Now you will be getting recoil so you can go for one of two recovery moves, Giga Drain and Pain Split. They both have their merits, Giga Drain does damage and can hit Ground, Rock, and Water types hard. Pain Split can work on anyone but works especially well on walls who generally have more HP. Focus Blast and Shadow Ball are your two main moves and provide good coverage beside each other.
I was thinking and I recalled one of my favorite OU sweepers, Gyarados. I'm going to give you a MoxieGem set that I particularly enjoy.
Gyarados @ Water Gem
Trait: Moxie
EVs: 252 Atk / 252 Spd / 4 HP
Jolly Nature
- Waterfall
- Earthquake
- Ice Fang
- Stone Edge/Dragon Dance
So Waterfall is your Main STAB and gets Moxie rolling with Water Gem, the rest is coverage although you can go for DDance to set up before you utilize the Gem.
There is the team with edits, I hope it helps and good luck in the future.