I keep having trouble finding mespirit. I have the tracker but every time..

I have a feeling that you used your masterball on palkia, am I correct? There are a couple ways that give you the best shot to catch mesprit. One is to find him many times and just slowly chip away at his health. The is to lead wih something that knows block or mean look so he wont be able to run away. As for finding him I don't really have any good advice, but don't fly to him because that resets it's position.

Don't fly get in between to routes and move back and forth till it's on you route(205 and valley windworks does good) try to attack with a Pokemon that has over 120 speed it's health stays the same then use a quick ball

For Locating Mesprit, you are on the right track. Just be patient and you will get in a fight with it again. The way I do it is to walk in and out of town until Mesprit comes close, then I pursue. For the actual fight, get a Golbat or Haunter with Mean Look and use it ASAP. This will keep Mesprit in the fight so you can weaken it. Either toss your Mater Ball if you didn't waste it already or Play at night to take advantage of Dusk Balls. Ultra Balls are always good to rely on. Weaken Mesprit to the red and paralyze or put him to sleep. Toss away while Golbat or Haunter are Mean looking and you should catch him eventually. Hope this helps. (I hate running Pokemon. Dogs and Latis started it)

DO NOT FLY!!!!!! Whenever you fly Mespsirit flies away. Hope this helps you out for cressecilia to.

I haven't met cresselia yet but for mespirit try using quick balls if that doesen't help try dusk balls at night but I found easiest to use Pokemon that know either block mean eye or migrate wobbuffet from r/s/e and use ultra balls DONT USE MATER BALL UNLESS DESPRATE because other legendaries have harder catch rate hope this helps.