* Weaken Mesprit before you try to catch it. You can do this by battling it, using status conditions, or using moves that lower its stats.
* Use a Quick Ball. Quick Balls have a higher chance of catching Pokémon on the first turn, so they're a good option for trying to catch Mesprit.
* Use a Timer Ball. Timer Balls have a higher chance of catching Pokémon the longer the battle has been going on, so they're a good option if you've been battling Mesprit for a while.
* Use an Ultra Ball. Ultra Balls are generally the most effective type of Poké Ball, so they're a good option if you don't have any Quick Balls or Timer Balls.
Remember, Mesprit is a rare and powerful Pokémon, so it may take a few tries before you're able to catch it. Be patient and keep trying, and eventually you'll be able to add Mesprit to your team!