Nincada evolves into ninjask at level 20, but you have to leave an open space in your party because after Nincada evolves, a Shedinja appears. My Shedinja appeared at level 27 with a max HP of 1. But this isn't a problem because Shedinja's "wonder guard" prevents any critical attacks.
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Once Nincada reaches level 20 it can evolve into Ninjask. However, if you have an open spot on your team another Pokémon Shedinja will appear also.
First, you have to have 1 empty slot in your party (make sure NINCADA is in it) and give NINCADA a regular Poke'ball. Level it up to Lv. 20 and let it evolve to NINJASK then when it evolves go check you party and SHEDINJA will be there for some reason...
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
Nincada evolves into Ninjask at level 20. If you have an empty slot in your party and an extra pokeball (a normal pokeball) Nincada will also evolve into Shedinja (It evolves into two Pokemon at once)
Nincada evolves into Ninjask at level 20 while Nincada evolves into Shedinja when you level it up while you have space in your party and your belt.