It wouldn't be "unstoppable"
Have you ever heard of SturdNinja? It's a niche strategy in Balanced and Pure Hackmons, which aims to do a similar thing, but it has Sturdy as an ability and the item is, usually, Protective Pads or Safety Goggles. Your Focus Sash would take the item slot but would act almost similar to the Sturdy ability, therefore we can draw parallels and can answer this question with SturdNinja in mind.
First of all, Shedinja will have loads of problems, which I'll showcase from Shedinja's Smogon Analysis.
It (Shedinja) suffers from very exploitable weaknesses. Shedinja's 1 HP leaves it KOed by any form of passive damage such as hazards, status, and weather. Photon Geyser, Sunsteel Strike, and Moongeist Beam ignore abilities, meaning they bypass Sturdy (Wonder Guard, over here) and are able to KO Shedinja. Mold Breaker users can bypass Sturdy and KO Shedinja with any attack, and if the Mold Breaker Pokemon has Pursuit, it can ensure a KO on Shedinja due to it being KOed if it attempts to switch out. Due to these weaknesses, Shedinja requires a lot of support to use successfully, such as Magic Bounce support and hazard removal. In addition to the aforementioned team support, Shedinja requires a lot of scouting to avoid being surprised by the above checks and getting KOed.
Almost all of Shedinja's issues are pointed above. To continue, once the Sash wears off, any Pursuit and/or any other move can easily kill Shedinja, not to say that it'd become deadweight for your team. Also, Ghost-types are immune to Endeavour, and with Shedinja's pretty low Attack stat, good luck doing only 30% to bulky Ghost-types, which are common in almost every tier. Weather damage will also make hay of the Shedinja because of its hard-coded 1 base HP stat. Some other weakness are below, courtesy of Smogon's Analysis again.
Moves That Ignore Abilities: Sunsteel Strike, Moongeist Beam, and Photon Geyser all KO Shedinja and can surprise it from any offensive Pokemon. For this reason, it's crucial to scout for these moves and potentially stall them out if necessary using Shedinja's teammates.
Mold Breaker: Mold Breaker attacks bypass Sturdy, so they OHKO Shedinja. Fortunately, Mold Breaker announces itself, so unless the Mold Breaker user has Pursuit, Shedinja can simply switch out. However, if Mold Breaker Pokemon have Pursuit, Shedinja has no viable way to beat them and inevitably will get KOed.
Status: If Shedinja is burned or poisoned, it will get KOed unless Shedinja holds a Lum Berry. It is recommended to scout for status. However, Shedinja can avoid being statused with Misty Surge support.
Ghost-types: Ghost-types can switch in on an Endeavor if necessary, and if Shedinja lacks Foresight, they can cause Shedinja trouble. Giratina in particular deserves a special mention, as even if Shedinja has Spectral Thief, Giratina is bulky enough to not get 2HKOed by it.
Sand and Hail: Sand and hail KO non-Safety Goggles Shedinja. There is no way for Shedinja to beat these two weathers other than Safety Goggles, so it is recommended that you KO their setter as early as possible using other teammates. Fortunately, these two weathers are relatively uncommon in the metagame.
Those weakness are pretty common and exploitable, making this strategy extremely niche. You're better off with some actual offensive Pokémon to dish out damage, every tier has some dedicated stallbreakers and wallbreakers.
Hope it helped!
