NOTE: no guarantee that it will work, but this is how I used to do it backin the day, I haven't gotten far enough with SS yet.
Get a Speed EV-trained Gengar or Haunter with Hypnosis and Mean Look and a pokémon (preferably Scyther or Scizor) with False Swipe.
The Haunter/Gengar should be below level 40, preferably between lvl 35 and 39, and all buffed up in speed. The levels of the other Pokémon don't matter.
1.Go to the route where Entei/Raikou is.
2.Put Haunter/Gengar first in your party.
3.Use (Normal/Super/Max)Repel (NOTE FOR NEWBIES: Repel keeps wild Pokémon of your level or lower away. Entei and Raikou are level 40, so if your leading Pokémon is level 40 or higher and you use repel, you won't encounter them. If you don't use Repel, you're likely to just encounter random wild Pokémon all the time.
4.Look for Entei/Raikou
5.When you find them, use Mean Look to keep em from running away, then hope they don't use Roar.
6.Now use Hypnosis to put Raikou/Entei asleep. (Keep your fingers crossed cause this move has low accuracy)
7.If it hits, switch to Scyther/Scizor and have it use False Swipe. This move will always leave at least 1HP left for the enemy.
8.If Raikou/Entei wakes up during step 7 and uses Roar, go look for them again, they'll still have the same amount of HP left.
9.Once they only have one HP left, start spamming Ultra Balls and hope you're lucky.