Things You'll Need
Instructions
Travel to Twinleaf Town and head north. Walk west along Route 201 and up north again to reach Lake Verity.
Approach the water and press "Start" to bring up the in-game menu. Select "Pokemon" and choose the Pokemon with the "Surf" ability to surf onto the lake.
Surf towards the cave in the middle of the lake and enter the cavern.
Approach Mesprit in the middle of the cave and press "A" to confront it. You will startle it and it will run away, but you will receive Mesprit's Pokedex entry.
Press "Start" and select your Poketech gear. Open the "Marking Map" application to track Mesprit's whereabouts in the wild.
Find Mesprit's current location on the map. Exit the cave, surf back onto the shores of Lake Verity and exit south onto Route 201.
Press "Start" and open your Key Items bag. Select your bicycle and bike towards Mesprit's location.
Walk or bike around in the tall grass where Mesprit is currently located to encounter it. If you encounter other wild Pokemon, either defeat them for experience or run away from battle.
Battle Mesprit and weaken its hit points into the critical red zone. Use status changing moves such as Hypnosis and Thunder Wave to make it easier to catch.
Throw your strongest Poke Ball (such as an Ultra Ball or a Timer Ball) at Mesprit to catch it. If it breaks free, continue throwing Poke Balls until it is captured.