Cresselia knows confusion, aurora beam, magical leaf, and slash
You can find Cresselia anywhere on the sinnoh map. WARNING: Cresselia moves around the map nonstop so it might be a little tricky to catch up to it. Don't check on your bag map, check on your app watch. Have fun!
Cresselia is always at a random spot on the map, and since every game is different, it is impossible to answer where Cresselia is next. You should get the Marking Map App for your Poketch, which gives the exact location of Roaming Pokemon (e.g., Mesprit and Cresselia).
After a little while into the post game, Cresselia starts appearing into your dreams or at least Darkrai is. He was pretending to be Cresselia. Cresselia is really good, but the Cresselia in the dreams are Darkrai's doing.
I looked it up, you can get Cresselia, but it doesn't say how. It only says you can get Cresselia at Sharpedo Bluff.
you find cresselia on full moon island
no, cresselia can only be caught in gens 4+
Well, Ice beam for a Choice Specs Cresselia and Icy Wind for a Cresselia with high stats on defense
If Cresselia disappeared off of your map completely, then you have already caught Cresselia, or there is a glitch in your game where Cresselia has just disappeared off of your map. Further details would be needed for a more in-depth assessment as to why this has happened if neither of the two reasons stated are why Cresselia isn't on your map.
because it was only one and not the whole colony of cresselia
You get cresselia eventually once you beat all the treetops.
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Cresselia doesn't evolve and nothing evolves into it. So its just one Pokemon.