How do you steal Pokemon from the Battle Factory in Pokemon Emerald?

The battle factory is the only facility where you can't use your own Pokemon, your given 6 Pokemon to choose from and you must choose 3 pick the best obviously. When you finish the guy brings you in the factory before facing your opponent he gives you a hint on there battle style or if they have certain Pokemon choose the 3 Pokemon that you have correctly to give yourself an advantage. When you defeat a trainer you have the option to swap Pokemon with that trainer this means if you found that they have a better Pokemon then you you can take it and swap it for one of yours that seems to be not working out for you.