To be extremely honest, Shedinja's a very gimmicky Pokemon at best. It won't live for more then maybe 3 turns, so honestly, I think it's got better things to do than use Toxic. Give it a Sash, and teach it Swords Dance, Shadow Sneak, X-Scissor, and Protect. I don't recommend using Toxic is because the turn that you do use it, the opponent could easily seed you, attack you supereffectively, etc.