After you get the National Dex go to a route(bring Max Repel, Master Ball and a pokemon that is higher Lv. than the wild pokemon but lower level than Raikou(Lv. 50) in front of your party)
1. Save in a route
2. Use Max Repel
3. Run around until Max Repel wears off
4. If you didn't see ;243; then soft reset and repeat the steps
This worked for me
Hope you get ;243;(i doubt you'll get one w/ good nature/IV's)