>>21964301>set 3DS clock to 9PM>buy like 20 Dusk Balls>put a Synchronizer in front for Nature>let it die, send out something tanky>smack Groudon into the red>start throwingI can't be bothered to spend days on getting a good Nature and IVs, but you can catch him pretty fast if you just use Dusk Balls, sleep optional. At least bring an Adamant Ralts with you or something with Synchronize, also. Huge cutscene afterwards, so you wanna get a good one in your first few tries.
