Somebody asked a similar question and here was my answer
Beat / Catch Groudon
Pretty simple right? Well here's the detailed walkthrough. (PS this could contain small spoilers)
1. Go to Sootopolis City and enter the Cave of Origin
2. Go to the other entrance (it should be on your left)
3. You should notice that the rocks that were there have disappeared allowing you to take a different route
4. Follow it down (it's a straight route so you won't get lost)
5. This will take you to he deepest part of the cavern where groudon is
6. The rest is up to you, faint or catch? (if you choose faint you can always catch it again using this walkthrough 24 hours after you fainted it
If this doesn't work you could always try the same route you took first (the one at the top) and groudon could take you to the deepest part of the cavern yet I don't know.
My last answer
There pretty much the same solution, follow those steps
