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Whenever I see Raikou's/Suicine's location I cut them off but somehow they get away.I never see them because whenever I get to the Cut-off point there already gone.
Everytime you go to the area Riakou/Suicune were, they run away.Go to a route where they aren't there(make sure that it wasn't one of their last two locations) and when you get there, there is a chance one of them could be there.
I forgot to mention that once you find them they run away when it's their turn to attack instead of attacking(you probably already know that) so you may want to bring a fast pokemon.Plus they both know Roar.It's a good thing that they never heal but that means if they faint they're gone forever.Although in Crystal you get a Clear bell in Goldenrod City and when you go to the Tin Tower you can fight Suicune and it doesn't know Roar plus it won't run away.And in Crystal after you get all 3 legendary beasts you can fight Ho-oh in the Tin Tower.
Well, I think the best way to get Entei is to go at Rout 43. Most of the time, the dogs usually go there but sometimes not. If you can't find Entei in that way, go to the rout abouve Olivine and beside Eurutek city.