Use repels, and bike back n forth between Santalune and route 4. Ride around in the flowers for a few seconds then back to town n repeat.
Took me about half an hour to encounter Moltres 8 times. Oh, and the 1st pokemon in your party has to be under level 65, otherwise it will repel the bird.
Finally something that works for me. Not that the others aren't viable, but this actually worked. Seen the bastard twice now. Can I use the stealth O-Power instead of Max Repels? --- 3DS Friend Code: 5472-7787-5897
I've seen him 11 times now and he's still appearing on the roads and not in his cave. This is by far the least fun I've had in this game. Hatching eggs for 2 days straight, 16+ hours was less frustrating than this.
Edit: Apparently it takes 12 times. Guide told me 10 times, but someone above said 12 so I suppose I should have read that. --- 3DS Friend Code: 5472-7787-5897
I got mine around 10. I've lost count actually so I don't know.
Max repel is better since it prompts you to use another one whereas using an O Power would have to make you click the PSS, choose O Power and scroll down a bit. Plus stealth only decreases encounter, doesn't eliminate it. --- 3DS - Pokemon Y - FC - 1993-8518-1210