1. Catch a Gastly at Sprout Tower.
2. Use Gastly as part of your team (eg. level him up and evolve him).
3. When you get to Blackthorn, you should have a Gengar.
4. Go to the move tutor and make Gengar remember Mean Look and Hypnosis.
5. Buy assloads of Dusk Balls from the guy near the Safari Zone.
6. Wait till night time.
7. Trap him with mean look, make him sleep, lower his health, and spam Dusk Balls. Gengar is hella fast so as long as the Mean Look hits first, which it will, you're good to go.
Caught Entei in 2 hours, used him to wreck kids at the E4.