With 1 HP, he's not good for much except for switching in to a move that he can absorb. However, he has a ridiculous move distribution. I'll pick the 4 worst, in my opinion, but Bulbapedia has his full moveset if you want to check more.
Final Gambit (Why, Game Freak, WHYYYYYYYYYYYYYYYYYY?) Sandstorm (Because he wants to CAUSE that...) Substitute (Because 1/4 of Shedinja's HP is a lot) Rest (To restore HP, you know)
A lot of choice here, huh?
Ah, the question: Toxic Orb, Flame Orb, or Life Orb?
With our set, the Life Orb will do nothing, and it's quite possible (I don't really know) that he might not take the poison damage, so let's give him the Flame Orb.
Can you think of any worse sets? Anything you'd change?
Gen VI in 2013? I'll believe that when Iris becomes Champion! Official Pokemon #678 of the Pokemon X boards. What?
Magikarp with only Splash is worse, because he's more susceptible to being set up bait. Your Shedinja will take itself out on turn one and allow for a better Pokemon to take its place. Magikarp will just sit their flopping around allowing the opponent to easily set up and sweep or for a stall team to get all its hazards down which is pretty much a guaranteed win as well.