The way I got my shiny charmander was by chaining 31 weedle and then going to route 3, using a mac lure and running from charmanders that weren't shiny. You can get approx. 400 encounters in an hour doing this, but the IVs won't be great and you won't get any charmander candy, but if you want the shiny for competitive it isn't too hard to get the right nature and grind to lvl 100 for bottlecaps
