My suggestion would be buying lots of Quick/Fast balls, and doing the old trick you use in the old games. If you can't afford the money, try to get a Pokemon with Mean Look. Based on what I've seen so far, Entei and Raikou DO NOT flee on the first turn, so even if you have a slow Pokemon with Mean Look, its fine. But it literally depends as I'm not 100% sure about it.
EDIT: Actually read about the fast balls and they actually work better on FAST Pokemons ( High Speed ). Mixed it up with the quick balls! P.S - To get a Fast Ball though, you can give Kurt white apricorns.