1 - You need a pokemon that learned "Thunder Wave" and that attack first when encountering Enter or Raiku.
2 - You need a pokemon that learned "Block" i suggest "Snorlax" that is lvl 50 and already know it when you get it in the story.
3 - 10 speed ball and 10 hyper ball eventually 10 shadow ball (when it's night)
With that you're ready to get these legendary's fleeing pokemons ^^ It took me about 15-20 mins to get these so it's really easy.
Go to the intersect of routes 38-39 on your bike and save the game. Each time you change route take a look on your map to see where it is and you will get Entei easily on the route you are. Don't use repels if your first pokemon is above lvl 40.
When fighting Entei use Thunder Wave the first time then he flee. Refight it with snorlax then use Block. After that he will flee eventually with using "Roar" but don't worry you have some turns to hit it to lower his HP.
You may use Speedball, and hyperball by alternating the choices. 1 speedball that don't work then use hyperball after.
You may capture it this way ^^ good luck !
Same way as Entei but go to the intersect of route 29-46 using your bike then save your game. You will find it easily on route 46.
Important: Don't forget to reset your DS if you used many balls and didn't caught it or you'll have to rebuy balls it prevent to loose money.
Good luck everyone ^^