me again

You should try walking to the location, and you just get lucky because each time you change route Mespirit does to.

Well, hello there Sammaslin, I hope that my answer is the answer that you are seeking, :D.

Well, you should listen to that friend of yours more often because he is right. If you catch and train a Pokemon that knows the move mean look, then you will be able to stop the legendary Pokemon Mesprit from running away in every battle. One Pokemon that I know learns the move Mean look is the ghost Pokemon, Haunter. If you want to know where you can find a Haunter in Pokemon Diamond, then just head on over to Serebii.net and have a look at their Diamond pokedex. When you get ready to capture Mesprit use a move such as Hypnosis or Thunder Wave. That way, you can either Paralyze Mesprit, or put him to sleep so that he will be a lot easier to catch.

Hope this helps.

Ok walk around near it. Go between each route and you'll see it moves everytime you switch routes. So, if you're close to it then keep switching routes and you might get lucky and be on the same route. Catching it is harder. Make sure you have a Pokemon that is faster than it is, one that can paralyze or sleep, or has mean look. Then you can catch it, most likely.

Whenever you reach a different route, it runs away to a different route. You must reach a route at the same time it does in order to battle it

I hope this helps