If you faint Cresselia in Pokémon Diamond or Pearl then you're out of luck and you'll have to either cheat for it or you'll have to trade with someone who has one however if you have Pokémon Platinum and you faint Cresselia then it'll regenerate so you can battle it again after you beat the Elite 4 and the Champion.
nothing
If you kill Garitina on diamond, you will kill am and never have him. i would recommend saving before you catch Garitina.
Nothing, you just can't catch him.
In platinum it would be better not to kill mesprit uxie and azelf, it would be better to catch them.
no it isn't once you kill or catch him you cant re battle him/her
It depends what game you have. In diamond/pearl you can't. In platinum, if you make it faint (not kill, Pokemon can't die) you cab't catch it, BUT you can catch again its its ALTERED FORM (which looks cooler).
You cannot catch them. However, you can still receive one in a trade - through a friend or over the WFC
Easy if u dont catch DIALGA and you kill it, it will say dialga vanish or soming
You wont be able to catch it ever again. I would suggest turning of your game then turning it back one. Then try to catch it again.
When you kill any legendary Pokemon (Mesprit included), you will not be able to catch it again. It will be gone forever. That iis why it is best to save before you battle a legendary.
Well, the game does not require you to catch Lugia, if you kill it you can still continue the game without it. However since it is a legendary, I think you should catch it. But you can do whatever you want, I'm just making a suggestion.
Yes you can rematch greevil at anytime and he will still have his shadow Pokemon if you defeated them.