Legendary Pokémon

What's the trick in Pokemon breeding in order to give a birth to a Pokemon with the gender I want (for example a female Mawile) ?

Unfortunately there isn't a way to define what gender the pokemon will hatch.Its based on its gender ratio and as far as i know there is no way(neither item nor ability) to change this in your favor.So for Mawile specifically the chances are 50-50.