If Cresselia faints [same as any legendary pokemon] you cannot catch it ever unless you save before the battle and restart if you make a mistake.
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There isn't actually a specific place were you catch cresslia. you first meet it at full moon island. then you go to your poketch,and if you have it, there is a program which will let you see were cresslia is wandering around in pearl. really after you first meet it at fullmoon island, the net time you see it will be somewere random.
it will be the female Pokemon
After you beat the Pokemon league then you beat the volcano after that you just train forever!!!!!!!!!!!!
Nothing, you just can't catch him.
Pokerus happens at random, it is about as rare a finding a shiny pokemon.