How do you catch crescelia without a master ball?

Well, she keeps her health just like Mesprit, so put a haunter/gengar at the front of your party (because those pokémon have real high speed) at a higher level than Creselia, so that it has a higher speed than Creselia. Therefore, your pokémon goes first. Give your haunter/gengar these following moves: Mean Look, Hypnosis, Dream Eater, and Night Shade. You use Mean Look first to make her not run away, then Hypnosis just as a precaution, then Dream Eater and Night Shade until she's down to red! (Ghost moves are super effective against her so watch her health closely) Once she's in red, throw a Poké Ball or whatever you have at her (you may have to throw several, so just use Mean Look every now and then) and then you've got her!

also you can use trapinch but dont switch it out and dont evolve it or you can use sudowoodo and use block.