Let's get a Mew!! Pokémon Luminescent Platinum

A Shroomish is near the start of Route 203 near some flowers and trees.
Talk to it with an Oran Berry in your inventory. It'll take it, transform into Mew and disappear.
After seeing the Shroomish part, you'll find a Buneary on Route 209 above a muddy slope.
Talk to it with a Sitrus Berry in your inventory. It'll take it, transform into Mew and disappear.
After seeing the Buneary part, you'll find a Torchic in Pastoria City on the alcove above the lake.
Talk to it with an Old Gateau in your inventory. It'll take it, transform into Mew and disappear.
After seeing the Torchic part, you'll find a Pachirisu on Route 201.
Talk to it and it'll transform into Mew, but stay in place.
Talk to Mew to encounter it.