I am very sorry but there is nothing you can do if you saved the game. If you didn't, then you will be able to encounter her the same way as earlier.
If you did safe the game after you fainted Cresselia, you may be able to persuade a friend to trade Cresselia with a Pokémon that you have caught.
If you encountered and defeated Cresselia in Pokémon, don't worry. Legendary Pokémon like Cresselia can often respawn after you defeat the Elite Four or in certain special circumstances. Keep playing the game, and you may have another chance to encounter Cresselia in the future.
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.
Cresselia knows confusion, aurora beam, magical leaf, and slash
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.
WHAT WORLD ARE YOU IN!!! Nowhere does it say Milotic evolves into Cresselia!