I would use a dusk ball.
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NO (only on ruby)
No you can only get groudon in Pokemon ruby or emerald and transfer through Pal Park or if you have heart gold or silver soul you can catch and trade groudon to yourself
you can`t catch kyogre or groudon or both of them without linking them from sapphire, ruby or emerald version.
The only way is if you saved your game before attempting to capture it and if you failed you can reset the game and try again other than that its not possible to retry catching groudon you can however take a groudon from emerald if you failed to catch it or another ruby game. So, no.
Nothing much would happen, but you must defeat it or catch it before you can battle the elite four