Shedinja is far and away the most unique Pokemon in the game. It has just 1, yes 1, HP, but it's protected by its ability Wonder Guard, which makes Shedinja completely immune to any damage that isn't a super effective attack or passive damage. While immunity to a whole slew of attacks sounds awesome and would make Shedinja one of the premier walls in the game, it is unfortunately KOed by literally any passive damage, which is rife in today's metagame. Shedinja's stats are also pretty unimpressive; it's extremely slow and has a terrible movepool which undermines its good Attack stat.
Shedinja is hard to use, but it does have a valuable niche in the metagame; it's one of the best counters available for Water-, Grass-, Poison-, and Electric-type Pokemon that lack Toxic or a super effective attack, which normally includes Pokemon such as Milotic, Venusaur, Leafeon, Exeggutor, Lanturn, Slowbro, Azumarill, Feraligatr, and Cloyster, among many others. Shedinja is the definition of high-risk high-reward, and if you can keep it away from Ghost-, Fire-, Rock-, Flying-, and Dark-type attacks, as well as burn, poison, sandstorm, hail, Leech Seed, and entry hazards, it can prove to be the ultimate "win condition" Pokemon.
This set tries to abuse Shedinja's decent base 90 Attack stat and mediocre movepool to its fullest. Using Shedinja is a very complicated ordeal, and the strategy of this set is actually not as straightforward as you'd think. Because there are so many Pokemon that counter Shedinja, the best move to use when you successfully get Shedinja in on a Pokemon that can't harm it, such as Venusaur and Azumarill, is to burn the switch-in with Will-O-Wisp, instead of using Swords Dance right away. The reason behind this is because the go-to counter for Shedinja for most players are Pursuit users, since they can KO Shedinja whether it stays in or not. Shedinja essentially has two lives with a Focus Sash, so after it (hopefully) burns the switch-in and gets hit with Pursuit as it switches out, Shedinja will have one life left to come in again, and the opponent's Pursuit user will be severely crippled by the burn from Will-O-Wisp. This will make it much easier for your team to dispose of the opponent's Pursuit user, and lets Shedinja come in later in the match to set up on whatever Pokemon are left on the opponent's team, without fear of getting trapped by Pursuit.
Shedinja is incredibly slow, so its main attack is left to a priority move. The choice between Shadow Sneak and Sucker Punch is entirely down to personal preference. Shadow Sneak is preferred though due to its consistency, and it is only slightly weaker than Sucker Punch thanks to STAB. X-Scissor is a nice powerful STAB move that grants Shedinja coverage on Normal-types. Shedinja should not begin to Swords Dance until you are confident you know the opponent's team and can predict the opponent's next switch-in. Once you are sure all threats are eliminated, Swords Dance to your heart's delight and watch as your opponent just succumbs to Shedinja's silent might.Team Options & Additional Comments >>>
There really isn't much else Shedinja can do. Life Orb and Substitute cannot be used, as the recoil from both will faint Shedinja. A Choice item + Trick set can be utilized, and can really mess with some of Shedinja's common counters such as Registeel that carry Shadow Claw, but Shedinja works much better with Swords Dance as an end-game sweeper. Shadow Claw is an alternative STAB move to Shadow Sneak, but Shedinja really wants priority to bypass its middling Speed. Aerial Ace is a nice move to put the hurt on Venusaur, Scyther, and Fighting-types, but there just isn't any room for it on the Swords Dance set. Shedinja is a good sleep absorber and can make use of Sleep Talk, but it can't use Rest. Finally, Toxic can be used instead of Will-O-Wisp, as it is a nice way to defeat Pokemon such as Milotic, but Shedinja's utility in burning physical attackers is too good to pass up.
Checks and Counters
Outside of entry hazards, poison, burn, Leech Seed, and damaging weather, Pursuit is the easiest way to dispose of Shedinja. Drapion, Absol, Skuntank, and Spiritomb are all good users of it, just be sure to avoid Will-O-Wisp when switching in (though the decreased Attack won't matter when facing Shedinja). Literally any Fire-, Rock-, Ghost-, Dark-, and Flying-type attack will KO Shedinja, it doesn't even need to be powerful, you just have to hit the husk. Finally, if all other options are exhausted, confusion is a great way to mess with Shedinja. However, if you lack any of the above options, the Shedinja user will win automatically unless you can somehow manage to PP stall it.