[Philosophy In A Teacup]
Mesprit is what we call a runner. Meaning, it appears randomly throughout Sinnoh once you "release" it and encountering it again is just luck. You can track it, and other runners, on the Mark Map App on your Pokétch, but it'll switch locations every time you do.
That's why most people use Master Balls on runners. They're a paaaain to catch unless you have something that knows Spore or Mean Look. True though this may be, there's more to running Pokemon to that.
First, 'Runner' Pokemon always flee battle the first chance they get--on the first turn, if possible. Second, they don't just appear randomly--they move from route to route; every time you change routes and locations, they do, as well. If you're clever and fortunate, you can plan out an interception course!
Beyond that, running Pokemon like Mesprit are often high enough in level that they 'stand out' from other Pokemon on the routes in question. You can take advantage of this! If you use repel, and your pokemon is lower or roughly equal to the level of the runner but higher than that of the running pokemon, it'll be the only encounter possible, ensuring you won't waste time on random encounters.
However, it's ALSO worth noting--and this is very important--that Running pokemon CAN NOT HEAL between battles. Nor do they recover from status ailments like Paralysis; it's really important that you recognize this. You can wear it down over time. Lastly: Running pokemon disappear after you encounter them, and reappear on a random route. IF YOU KNOCK IT OUT, IT NEVER RETURNS! BE CAREFUL! It's best to have a pokemon who's really fast whittle its HP to one with False Swipe one encounter before you go for the capture.
Good luck, and good hunting.
(And that's all I have to say about that~)