Mesprit

Mesprit is the hardest of Uxie, Azelf, and Mesprit. the reason it's hard is that it runs around the Sinnoh region. Once you get in a battle, make sure you have a crobat around lvl. 50 with mean look so mesprit doesn't run away. make sure you have lots of full restores, because if crobat gets switched out, or gets K.O. then mesprit is free to run. you find it in Lake Verity. once you click on it, it will start running. you use your tracking map poketch to find it and do not use your bicycle, otherwise it will always run. and try not to fly to it. in order to catch it easier, go to the upper right corner of jubilife city, and walk beetween the route and the city and mesprit will keep running. wait 'till it comes near you and the walk to it. make sure crobat is number 1 in your party so it can use mean look at the start.

buy about 20 ultraballs and 50 timerballs. all you need is some time to catch it.

His stats are:
Pokedex No. 481
Type: Psychic
Lv.: 50
HP: 142
Attack: 100
Defense: 111
Sp. Atk: 123
Sp. Def: 119
Speed: 102
Ability: Levitate. Gives full immunity to all Ground-type moves.
Moves: Swift PP 20 (Normal), Lucky Chant PP 30 (Normal), Future Sight PP 15 (Psychic), Charm PP 15 (Normal)
Report on moves: Swift. Power: 60, Accuracy: ---. Star-shaped rays are shot at the foe. This attack never misses. Lucky Chant. Power: ---, Accuracy: ---. The user chants an incantation toward the sky, preventing the foe from landing critical hits. Future Sight. Power: 80, Accuracy: 90. Two turns after this move is used, the foe is attacked with a hunk of psychic energy. Charm. Power: ---, Accuracy: 100. The user charmingly stares at the foe, making it less wary. The target's Attack is sharply lowered.