Well. No it wouldn’t work. Not even close.
First off, in the case that you were attempting this in a Doubles match, you would have to first pull off the Carbink (That’s the Rock/Fairy Type) Skill Swap to a Level 1 Smeargle, and follow that up with switching out Carbink for Shedinja. This means you give your opponent the chance to hit you with an attack, therefore killing smeargle, or Shedinja if it so happens to be super effective. Additionally, they could try setting up hazards, and causing Shedinja to die immedietly upon switching in.
Even in a triple battle, the point is you’re using a LEVEL 1 SMEARGLE. Do you know how weak that is? A level 5 Sunkern OHKOs it with Earth Power. And that’s not even STAB. Though Sunkern is god. Anyway, that means you can’t take a hit – any kind of hit. Unless it’s Splash Magikarp. Or False swipe. You get the point. So Smeargle is going to die before it manages to do jack, therefore making your whole strategy go down the drain.
However, lets just assume for one moment, the opponent decided to set up and let you pull off such a combo. Then you’d have to deal with a couple more issues even with a Sturdy Shedinja. Moves like Leech Seed, and item effects like Rocky Helmet, and statuses like Burn and Poison, which all do damage and will hit through the effects of Sturdy, will all still kill you – since none of them are actually damaging attacks. Additionally, Pokemon with the ability ‘Mold Breaker’ or any of it’s variants will be able to bypass the effects of sturdy, and kill Shedinja straight away. Good job Shedinja, you’re dead now. The move ‘Taunt’ also destroys this strategy. You also forget, the phasing moves Whirlwind and Roar can throw Shedinja out the window, and cancel the effects of Entrainment, therefore bringing Shedinja back it’s original ability Wonder Guard – and then the opponent could kill Smeargle and Carbink leisurely, therefore making you lose a couple Pokemon on your team for no real reason whatsoever.
Anyway in conclusion, no this strategy is far from Underfeatable, and probably shouldn’t be attempted.