When i finally have a female Gardevoir on my team, i tend to box it forever in the day care. Usually, I put a Muk or a Swalot there with it, so that it gets raped until i finish the game.
I remember a time where I got a shiny because I was repeatly doing this. I leveled the shiny until it became a gardevoir, and switched its mom with itself. I released the mom and kept its daughter in the day care, raped forever by some random Muk i catched.