I have two questions. I saw cresselia, but it ran away. Where can I..

Some wild sneasles will be holding something called a razor claw, if you find one that is holding razor claw make it level up 1 level between 8pm-4am,and it will evolve.

You first must track down Cresselia using you Marking App on your Poketech. Carry Pokemon with MeanLook, Spider web, Block, or a Wobbuffet. Weaken Cresselia when you find it and make sure moves like Mean Look is applied, since Cresselia runs at the first sign of a fight. Use Ultra Balls and DuskBalls If you are playing at night.
As for evolving Sneasel, just level him up at night while he is holding the razor claw and he should evolve into Weavile.

I suggest you go to the windmill place near Flamora. Keep on going back and forth and eventually it will be there.

Use the Marking Map app that you get from the Poketch Company after 3 badges to track it down. Your Sneasel will evolve you make it hold a Razor Claw, which you can find in Victory Road and it gains a level

Got to flamora town and keep on crossing the line betwwen valley windworks and the route next to it. It will eventually come in the area.

You remember the Poketch App Marking Map right? Number 13? If you move a little bit, a Pokemon head will appear and move if you change locations, whether you Fly to town or change Routes. Track the head until it's in your area and repel the grass, as well as having a Pokemon below Lv 50 first. I suggest using the Master Ball on Cresselia here.

If you have the marking map for the poketch the cresselia will be on the map all you have to do is go to where one route ends and another starts and ride back and forth everytime you switch a route cresselia will move