Basically, any attack that isn't super effective against Shedinja's Bug/Ghost typing just slides off because of its special ability, Wonder Guard. The only types that can hurt it are Fire, Rock, Ghost, Flying, and Dark. This is normally balanced by the fact that Shedinja only has a single hit point. It's literally just the shed skin of another Pokemon, so it's hilariously fragile. It's a gimmick Pokemon and the only one that can have that ability.
But if you take Wonder Guard and slap it on something that isn't overwhelmed by a single attack than you've hit the superpower lottery.
It only, however, blocks damage. And only direct damage at that. You can still affect Shedinja's stats, and indirect attacks like poison powder or sandstorm go right through it.
Hey son, wanna' learn how ta' make witch balls?