Okay here's a lil strategy for sure catch ;)
What you need:
- A Haunter/Gengar (in my case, Boo) that knows Shadow Ball (TM from Morty), Mean Look and Hypnosis. (I only had Haunter.)
- A Scyther with False Swipe. (Both my Haunter and Scyther were at 34, I did this right after I beat Jasmine.)
- Lotsa Poke/Great/Ultra Balls and Repels.
Okay so I stood between routes 37 and 36, which is right below Ecruteak, and east of National Park. (You should know that that's the right spot if you're beside a Trainer Tips sign that explained about the move Dig.) Then I put Boo first in my party. Time to track down the beasts! So go down to route 36, check the Pokégear. If any of them are not beside Nat'l Park, go up to route 37, then check the Pokegear. So how do you know if you're on the same route as them? Your character's icon has a black border, so if you see in your Pokegear that there's some white around your icon, one of them is probably there.
Note: In route 37, the patch of grass where they appear is the one below the 3 Apricorn Trees. In route 36, the patch of grass where they appear is the one right beside School Kid Alan. If you don't know where that is, from Sudowoodo's spot, go down then left. :P
When on the same route, use a Repel then go on grass. You'll probably encounter only Raikou or Entei with repels on. I used Mean Look to trap them, then Hypnosis for sleep. Then I spammed Shadow Ball, around 3 times, until they're in the red zone. I tried throwing some Pokeballs while it's asleep, but when it woke up, my Boo always got knocked out. So I sent out Scyther, and used False Swipe before it ran. I did this for the two of them, that leaves them both with 1 HP.
Then I kept tracking them between routes 37 and 36, then when I saw them with 1 HP, I used Mean Look, Hypnosis. Then spammed balls.
So this strategy was so effective for me, I caught both of them within an hour.