I've progressed to Mossdeep and defeated Tate and Liza, here is what I've found so far.
Shedinja fairs awfully against both Team Magma and Aqua, due to a large amount of dark types, alongside things like Zubat/Golbat. It also has poor gym match-ups, being unable to do much of anything against Winona since all 5 of her pokes pack aerial ace (Although it can switch into Tropius's Solarbeam if your desperate I guess). Against Tate and Liza only Claydol and Solrock can actually damage Shedinja, but since this is a double battle it's a lot harder to use Shedinja. However, Shedinja's Shadow Ball does do a lot of damage to their whole team, so if you can eliminate the threats to Shedinja with another Pokemon, it can contribute decently. But overall it doesn't perform too well. Lastly against May Shedinja fails to wall most of her team, but can wall Lombre in the 4th match (But not in the 5th), alongside Tropius in the 5th but that's about it (Unless May has Marshtomp, it can wall Marshtomp too). Against standard trainers Shedinja's match-up varies heavily (usually dependent upon which trainer class your facing), this gives it some matches where it solos because of Wonder Guard, and others where it just gets OHKO'd straight away.
Shedinja fairs awfully against both Team Magma and Aqua, due to a large amount of dark types, alongside things like Zubat/Golbat. It also has poor gym match-ups, being unable to do much of anything against Winona since all 5 of her pokes pack aerial ace (Although it can switch into Tropius's Solarbeam if your desperate I guess). Against Tate and Liza only Claydol and Solrock can actually damage Shedinja, but since this is a double battle it's a lot harder to use Shedinja. However, Shedinja's Shadow Ball does do a lot of damage to their whole team, so if you can eliminate the threats to Shedinja with another Pokemon, it can contribute decently. But overall it doesn't perform too well. Lastly against May Shedinja fails to wall most of her team, but can wall Lombre in the 4th match (But not in the 5th), alongside Tropius in the 5th but that's about it (Unless May has Marshtomp, it can wall Marshtomp too). Against standard trainers Shedinja's match-up varies heavily (usually dependent upon which trainer class your facing), this gives it some matches where it solos because of Wonder Guard, and others where it just gets OHKO'd straight away.