Question

How do I obtain Ash-Greninja (in ORAS, for example)?

Answer 1

You cannot obtain Ash-Greninja in any Pokemon game older than Sun and Moon. As, for getting Ash-Greninja in Sun and Moon, you have to complete the demo of Sun and Moon. After you complete the demo, you can transfer Ash-Greninja to the full game.

Source

Comments

How long does it take to complete the demo?

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If my memory is correct, it didn't take very long to beat the demo. I'd say maybe an hour or two, but probably less.

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Got him. Cool. Thanks for your help!

Answer 2

The only way to obtain a Greninja with Battle Bond is by playing the Sun and Moon demo, which can be downloaded for free in the EShop. You cannot get this Greninja in XY or ORAS.

Source: knowledge and experience