Just train. It'll help you a lot. Most of the time (unless you're recruiting Skarmories from Western Cave or something like that, or recruiting legends) Pokemon don't come strong. Your level plays a part in recruiting as well- the higher it is, the more likely a Pokemon is to join you.
And, as Thunder Beam said, you can't recruit most final evolutions such as Ampharos, as even with a Friend Bow (which increases the chance that a Pokemon will join) their recruit percentage is impossible. With a Friend Bow some of them have a very tiny chance (like Golem and Blastoise), but it's not big enough to worry about. I went to the final floors of Buried Relic, which have nothing but Golem, with a Lv.100 Leader holding a Friend Bow, and still never managed to recruit any.