My best OU team so far

Jirachi @ Leftovers
Trait: Serene Grace
EVs: 252 Spd / 252 SAtk / 4 Atk
Hasty Nature
- Thunderbolt
- Psychic
- Water Pulse
- Fire Punch

Infernape @ Electric Gem
Trait: Iron Fist
EVs: 252 Spd / 252 Atk / 4 SDef
Jolly Nature
- Flamethrower
- ThunderPunch
- Close Combat
- Stone Edge

Zoroark @ Flying Gem
Trait: Illusion
EVs: 252 Spd / 252 SAtk / 4 Atk
Hasty Nature
IVs: 30 Atk / 30 SAtk / 30 Spd
- Flamethrower
- Foul Play
- Shadow Ball
- Aerial Ace

Haxorus @ Persim Berry
Trait: Mold Breaker
EVs: 220 Spd / 252 Atk / 36 HP
Jolly Nature
IVs: 30 SAtk / 30 SDef
- Earthquake
- Brick Break
- Outrage
- Dragon Claw

Rotom-Wash @ Leftovers
Trait: Levitate
EVs: 252 SAtk / 252 HP / 4 SDef
Modest Nature
- Thunderbolt
- Hydro Pump
- Pain Split
- Shadow Ball

Jellicent @ Leftovers
Trait: Water Absorb
EVs: 252 SAtk / 252 HP / 4 SDef
Modest Nature
IVs: 30 Atk / 30 Def / 30 SAtk
- Scald
- Ice Beam
- Giga Drain
- Shadow Ball

So, as the title says this is my best OU team so far.It wins 8/9 battles which isn't so bad.Any suggestions on how to make it better?

Mixed Jirachi is better off as a SmashPass recipient, IMO. But if you want to keep it mixed, that's fine, but it needs better coverage options:
-Iron Head
-Fire Punch
-Hidden Power (Ice)
-Energy Ball
Nature: Hasty
EVs: 152 Atk/204 SAtk/152 Spe
Item: Expert Belt

or
-Substitute
-Calm Mind
-Thunderbolt
-Flash Cannon/Psyshock/Water Pulse
Nature: Timid
EVs: 252 HP/104 SAtk/152 Spe
Item: Leftovers

or
-Iron Head
-Body Slam/Thunder Wave
-Wish
-Stealth Rock/Protect/U-turn
Nature: Careful
EVs: 252 HP/236 SDef/20 Spe
Item: Leftovers

or
-Iron Head
-Ice Punch
-Fire Punch/Healing Wish
-U-turn/Trick
Nature: Jolly/Adamant
EVs: 252 SAtk/4 SDef/252 Spe
Item: Choice Scarf

or
-Wish
-Calm Mind
-Thunderbolt
-Flash Cannon/Psyshock/Water Pulse
Nature: Bold
EVs: 252 HP/236 Def/20 Spe
Item: Leftovers

or
-Substitute
-Body Slam/Thunder Wave
-Iron Head
-Fire Punch/Zen Headbutt
Nature: Jolly
EVs: 252 HP/160 Atk/96 Spe
Item: Leftovers

Jolly lowers Infernape's Special Attack making Flamethrower less useful. Gems aren't useful except for Flying Gem + Acrobatics, IMO:
-Flare Blitz
-Close Combat
-U-turn
-Mach Punch/ThunderPunch
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Choice Band

or
-Fire Blast
-Close Combat
-Grass Knot/U-turn/ThunderPunch
-Stone Edge/Mach Punch/ThunderPunch
Nature: Hasty/Naive
EVs: 252 Atk/4 SAtk/252 Spe
Item: Life Orb

or
-Fire Blast
-Close Combat
-U-turn
-Hidden Power (Ice)/Stone Edge
Nature: Hasty/Naive
EVs: 108 Atk/148 SAtk/252 Spe
Item: Life Orb

While Foul Play's 95 base power seems tempting, don't bother with it because it uses the opponent's Attack rather than Zoroark's to calculate damage (it's better on Pokémon that lack offensive presence, like Sableye or Umbreon, IMO). Also, its Special Attack is rather good, so why aren't you using Dark Pulse/Night Daze?:
-Dark Pulse/Night Daze
-Flamethrower
-Focus Blast
-Sucker Punch
Nature: Naive
EVs: 4 Atk/252 SAtk/224 Spe
Item: Life Orb

or
-Dark Pulse/Night Daze
-Flamethrower
-Focus Blast
-Nasty Plot/Trick/U-turn
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb/Focus Sash (former 2 for Nasty Plot)/Choice Specs/Choice Scarf (latter 2 for Trick/U-turn)

Haxorus gained more coverage options in B2/W2 in the forms of Superpower, Low Kick and Aqua Tail. Also, Lum >>>> Persim, as Lum cures any status Haxorus takes and confusion isn't the only status that can screw it over:
-Swords Dance
-Outrage
-Superpower/Low Kick
-Substitute/Aqua Tail/Earthquake
Nature: Adamant
EVs: 4 HP/252 Atk/252 Spe
Item: Fighting Gem/Lum Berry/Life Orb
Ability: Mold Breaker

or
-Outrage
-Superpower
-Earthquake
-Aqua Tail/Dual Chop
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Choice Band
Ability: Mold Breaker

or
-Dragon Dance
-Swords Dance
-Outrage
-Low Kick/Earthquake
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Lum Berry/Draco Plate

or
-Dragon Dance
-Outrage
-Low Kick
-Earthquake/Taunt/Dragon Claw/Dual Chop
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Lum Berry/Yache Berry
Ability: Mold Breaker

I feel that Rotom could be a bit faster. Also, Shadow Ball is rather useless on it as a neutral STAB Hydro Pump does more than an unSTABed supereffective Shadow Ball:
-Volt Switch
-Hydro Pump
-Will-o-Wisp
-Pain Split/Hidden Power (Ice)
Nature: Modest
EVs: 232 HP/68 SAtk/208 Spe
Item: Leftovers

or
-Volt Switch
-Hydro Pump
-Thunder Wave/Will-o-Wisp
-Pain Split
Nature: Calm
EVs: 248 HP/28 SAtk/232 SDef
Item: Leftovers

or
-Volt Switch
-Hydro Pump
-Hidden Power (Ice)
-Trick/Thunderbolt
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Choice Specs/Choice Scarf

I think Jellicent should be Choiced if it runs an offensive set as it lacks power somewhat:
-Water Spout
-Hydro Pump/Surf/Scald
-Shadow Ball
-Ice Beam/Giga Drain
Nature: Modest
EVs: 172 HP/252 SAtk/84 Spe
Item: Choice Specs
Ability: Water Absorb

Or it can go defensive, since Jellicent is primarily a defensive 'mon:
-Scald/Surf
-Will-o-Wisp
-Recover
-Taunt
Nature: Bold
EVs: 248 HP/216 Def/44 Spe
Item: Leftovers

or
-Scald/Surf
-Will-o-Wisp
-Recover
-Shadow Ball/Ice Beam
Nature: Calm
EVs: 252 HP/36 Def/220 SDef
Item: Leftovers
Ability: Water Absorb

If I may add, you're kiiind of weak to a DD Haxorus, because see - it can set up on your Rotom-W, and can pretty much OHKO the majority of your pokemon with Outrage and Earthquake. Sure, Jirachi can be a saving grace, but you should have added incentives to prevent yourself to be out sped and swept by +1 dragon pokemon.

What you can probably do is change Jellicent to a defensive orient, with a bold nature, and 248 HP / 216 Def / 44 Spe in EVs. It can also have moves such as Will-O-Wisp for a nice cripple effect. You can also try a more defensive version of Jirachi, that's careful natured and with 252 HP / 224 SpD / 32 Spe in EVs. It can have moves like Thunder Wave where it can cripple fast pokemon safely. Plus it can have Wish where it goes nicely with your Jellicent, because Jellicant can tank those fire moves from Heatran as it recieves nice wish passes from Jirachi. Your Rotom-W can also fit the bill on this too.

But, whatever works for you! I hope this helps, and good luck on battling~ ^^

Thanks for the suggestions guys.This is what i went with:

As I said before, mixed Jirachi needs better coverage options. I'll explain why each move is important. Iron Head's increased flinch chance courtesy of Serene Grace is too good to pass up. Fire Punch (or Drain Punch) lets Jirachi hit Steels hard. HP Ice lets Jirachi hit Dragons. Energy Ball lets Jirachi beat bulky Waters. This EV spread is better: 152 HP/204 SAtk/152 Spe.

Shadow Ball is redundant with Zoroark's Dark Pulse as both hit the same types for supereffective damage. As a rule of thumb, never use Ghost and Dark moves together unless it's for utility purposes (Spiritomb runs Shadow Sneak, Pursuit and Sucker Punch in the same set to let it trap Psychics and Ghosts). Since you're using a Life Orb set, the last slot should go to Sucker Punch to let Zoroark hit fast and frail revenge killers hard or possibly KO them; you must switch to a Hasty or Naive nature if you opt for that. Or if you're feeling gutsy or if you have the right Pokémon for Illusion (like Infernape), the last slot can also go to Nasty Plot to make Zoroark stronger than it already is.

What makes Specs Jellicent so good is Water Spout; it should never go without the move if it goes on the offensive. Assuming Jellicent is healthy, a 150 BP STAB move is too good to pass up. Scald, Surf or Hydro Pump mainly serves as a backup STAB if Jellicent's HP gets too low. To replenish Jellicent's HP, you can just switch it into a Water move to activate Water Absorb. Use it over Giga Drain or Ice Beam. The preferred EV spread for Jellicent is 172 HP/252 SAtk/84 Spe to let it outspeed Skarmory so that it can hit it hard with Water Spout.

Thanks again.

Jirachi @ Expert Belt
Trait: Serene Grace
EVs: 152 HP / 204 SAtk / 152 Spd
Hasty Nature
- Iron Head
- Drain Punch
- Hidden Power [Ice]
- Energy Ball

Infernape (M) @ Life Orb
Trait: Iron Fist
EVs: 252 Atk / 4 SAtk / 252 Spd
Naive Nature
- Fire Blast
- ThunderPunch
- Close Combat
- Stone Edge

Zoroark (M) @ Life Orb
Trait: Illusion
EVs: 252 SAtk / 4 SDef / 252 Spd
Timid Nature
IVs: 30 Atk / 30 SAtk / 30 Spd
- Flamethrower
- Dark Pulse
- Focus Blast
- Nasty Plot

Haxorus (M) @ Lum Berry
Trait: Mold Breaker
EVs: 4 HP / 252 Atk / 252 Spd
Adamant Nature
IVs: 30 SAtk / 30 SDef
- Earthquake
- Superpower
- Outrage
- Swords Dance

Rotom-Wash @ Leftovers
Trait: Levitate
EVs: 232 HP / 68 SAtk / 208 Spd
Modest Nature
- Volt Switch
- Hydro Pump
- Pain Split
- Will-O-Wisp

Jellicent (M) @ Choice Specs
Trait: Water Absorb
EVs: 252 SAtk / 172 HP / 84 Spd
Modest Nature
IVs: 30 Atk / 30 Def / 30 SAtk
- Water Spout
- Ice Beam
- Scald
- Shadow Ball