Seems weird to me that you're Baton Passing QD boosts to one Pokemon. I'm not good but I think you should change Quiver Dance to Shell Smash so you can have the option to pass it to three threats (Greninja, Garchomp, Excadrill)
That's just me, sorry if this advice seems bad.
Shell Smash does seem better, but I'm kinda iffy because a lot of my threats are already pretty frail, and if I'm Baton Passing to something that's gonna get hit I want it to have at least a bit more SpDef and not less.
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Quiver Dance => Shell Smash on Smeargle Shell Smash gives you more options, and generally more offensive prowess. In one of the videos you were able to set up multiple quiver dances. A Mega Garchomp at +4 with +4 speed is just insane. Greninja and Excadrill would both also hit incredibly hard.
Without more text, or more replays to get an idea of how members that aren't Greninja or Smeargle play it is kind of hard to rate the rest of your team.
Because of Garchomp's awkward speed tier, the only things you are outspeeding are: >Jolly Mamoswine, Dragonite >Jolly/Timid Lucario >Adamant Haxorus >Adamant/Modest Volcarona, Salamence, Jirachi Mamoswine will probably Ice Shard, Lucario is often Adamant (relying on priority). Salamence is normally Jolly/Hasty, whilst Volcarona is something that pretty much always sets up by boosting speed. Jirachi, if carrying max speed, is normally scarfed.
As such, you may want to try out adamant.
You could try out a Smooth Rock on Hippowdon, and Sand Rush on Excadrill. It also would allow Hippo to support Mega Garchomp.
Finally, Expert Belt => Life Orb on Greninja. There is a lot you don't hit super effectively, and Greninja needs all of the power it can get.
A minor tweak is Technician => Own Tempo on Smeargle. There's very little reason to run Own Tempo, since confusion is a rarity but there is zero reason to run Technician. It may help one day. *shrugs*.
Garchomp @ Garchompite Ability: Rough Skin EVs: 4 HP / 252 Atk / 252 Spd Adamant Nature - Dragon Claw - Earthquake - Fire Fang - Swords Dance
Smeargle @ Focus Sash Ability: Own Tempo EVs: 252 HP / 4 SDef / 252 Spd Timid Nature - Spore - Sticky Web - Shell Smash - Baton Pass
Hippowdon @ Smooth Rock Ability: Sand Stream EVs: 252 HP / 4 Atk / 252 Def Impish Nature - Earthquake - Slack Off - Whirlwind - Stone Edge
Greninja @ Life Orb Ability: Protean EVs: 4 HP / 252 SAtk / 252 Spd Timid Nature - Surf - Dark Pulse - Grass Knot - Ice Beam
Revised importables. I didn't post Excadrill's there, since your set has a lot of merit to it too.