In FireRed, it would probably be smartest to go after a Scyther in Celedon City's game corner. You just need to get hella coins then you save in front of the lady who asks you what prize you want, stock up on Scythers, check to see if they're shiny, if they are; great! if not... reset. I did that hunt for like two days - but I decided to move on to "Clefarios" and then I dropped that because I realized that if I actually wanted to make a balanced shiny team - having a Tauros would do me a whole lot better.
And so the hunt continues...