1) Select the poketech app that tracks the pokemon's location on the map. 
2) Be prepared, have the type of pokeballs you want. I like ultraballs and timer balls for
cresselia because it takes a while to catch. 

3) Fly to JUBLIFE city. 

4) Head right to route 203. 

5) Save your game in case you KO the running pokemon 

6) Now, in case you haven't noticed this, these running pokemon change location every time
you do. when you go from jublife city to route 203, mespirit/cresselia's location will
change as well. 

7) Head back for jublife city, and the pokemon's location will change. Go back to route
203, the pokemon's location will change again. All you have to do, is keep going back and
forth until the map shows the pokemon's location on route 203. When it does, go into the
grass, it is not far, just a little north of you. 

8) Depending on your luck, you'll be in the same place as this pokemon in < 1 - 5
mintutes, it's only a matter of time. 

Enjoy. I caught both mespirit and cresselia using these methods in a insanely short amount
of time.