your party needs to have room for a 4-star pokemon. that means you can battle with either
two 1-star pokemon or one 2-star pokemon. (you can see their size on the summary screen).
if you are using a larger party say farewell to your extra pokemon home 
 the higher your leaders level the the better your odds.
 your leader must strike the final blow in the boss fight and be adjacent to the pokemon
you are trying to recruit.
 get the friend bow for higher odds (bring a key with you to 30F of mount faraway)
i really hope this helps you 
Thanx 4 reading<table class="table_ch"></table>