You can cheat.Or if you don't wanna cheat its really hard but follow these steps:
1.After you find out where groudon is,save the game out of the cave.
2.Put the Magma Emblem in the 'Select Mode' and use it (there's only a 1% chance that it will work).
3.If it works,go inside.If it doesnt work,turn off the power and still try.
4.Once you've done all that,save the power.Go inside and see if its a shiny groudon.If it isn't,keep on trying.(It is a 1% chance that'll work as well).
If you don't wanna cheat or do all that,trade with someone who has done so.
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Research it up on www.Pokemon.Marriland.com to get the answers.
Of course find groudon in terra cave save before attempting to capture it then when you see it if its not a brownish color keep resetting the game until its shiny.
Go to Gym Battle, Beat the Hoenn Elite 4 and Sinnoh Elite 4 two times.And then click Map 8 and then click map 10 and go to your profile , and go to map 10 and then go to the middle of the map.then you will see Groudon, if you see Groudon kill it.then go to your Pokemon then you will see 1 normal groudon and 1 shiny groudon, Good Luck..
you beat the elite four first and rayquaza, groudon and kyogre will turn shiny No that is wrong you cant just beat the elite four an then the legendary Pokemon will turn shiny you either have to use a cheat system like action replay or gameshark or as soon as you get to the legendaries save your game an if the Pokemon is not shiny reset you game repeat this until it is shiny
You get RAYQUAZA at the top of SKY PILLAR after seeing GROUDON, KYORGE and RAYQUAZA in SOOTOPILIS CITY. RAYQUAZA is LV.70 so be prepared! RAYQUAZA knows these techniques: EXTREMESPEED REST FLY DRAGON CLAW. The ratio of it being shiny is 25%. The only difference between SHINY & NON-SHINY is that the SHINY one is BLACK, not the regular GREEN. The best ball to use on RAYQUAZA is either an ULTRA-BALL (Or if you are lazy and not bothered a MASTER-BALL!)