Well if your trying to find it, just chase it, but don't use fly, that will make Mesprit teleport to a random location, so, just run through the cities and routes, and don't use your bike, Mesprit will travel faster if you use your bike, so only use it if you have to. It took me about a minute and 30 seconds to find it just by running, and using my bike twice to get on top of sand slopes. But when it comes down to catching it, thats your call, I just migrated 6 Pokemon from my emerald with each of them holding masterballs, so I used a masterball on it! ^^ Also, this goes for the same with Creselia too, also to make this much easier, get the Poke'Tech APP called the Marking Map, which can be obtained at the Poke Tech Company in Jubilife City. So, I hope this helps you a lot. ^_^