You don't. You must evolve nincada in ninjask, but when you evolve it, have an empty slot in your party. After nincada evolves, check your party, 100% chance you'll have a shedinja
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it evolves into ninjask/shedninja. (you can only get shedninja if you leave a slot open in your party when you have nincada.)
nincada evolves at level 20 (i think) if you have an empty pokeball when you evolve nincada you will get a shedninja. nincada evolves into ninjask
I don't really know some other rare Pokemon but I do know how to get shedninja in Pokemon emerald. First, you have to find a nincada that has a bold nature and evolve it into ninjask. Right after it evolves, look in your party and you might have a shedninja that says you met it at the same level in the same area at the same level caught with the same kind of pokeball, the same ID number as ninjask, and even the same move set as ninjask. What I think is really weird about shedninja is he only has 1hp and it never gets more hp as you level it up. Hope this helps.
Ninjask cannot evolve any further. If you are meaning Nincada, it can evolve into Ninjask at level 20, but it can also evolve into Shedinja, if you have an empty party slot and a Pokeball with you.
ninjask doesn't evolve but if you have a empty slot of your Pokemon team when nincada evolves to ninjask a shedinja will appear in the empty slot.