It goes away forever! Unless you trade for it...
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Yes you can! After he faints go back and he'll be there.
Easy if u dont catch DIALGA and you kill it, it will say dialga vanish or soming
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.
U can't
After the battle ends the Pokemon will still evolve however you will have to revive it if you still want to use it.