I use this part of Perfect Guide strategy...
... you can select the "Area" command in its Pokedex entry to see its current location...
... Fly to a central location, like Violet City, and see if they are a route or two away. If theyare, move in their direction and hope they are still there. If they're not, just Fly back to Violet to shuffle up their locations...
... Use Sleep. Moves like Mean Look are tempting, but Entei and Raikou know "Roar" to end fights like that....
The guide says to use Fast Balls, Heavy Balls, and Friendship Balls. WTF???