Requirements: National Dex, Beat E4, Access to Buck's grandpa's club.
If you have pokemon you want to level why not battle those gym leaders again in the club But you have to go to bed... Have no fear! Save in the club with fully healed pokemon and if you have to use exp share then let it be i suggest using a lvl 100-noob pokemon /w exp share. The next day when u turn platinium on u will still be in the club with the same leaders beat em with your high level(If you choose that path) without going outside to heal. Once you beat them all save if you wish then exit and head to the pokemon center to heal if you have to. Then go back into the club. You will find different Gymleaders/ Trainers then u battled before, (Might have one same one that will still battle you. Untested) Once you battle them your done for the day. But you might be able to do it every 1-2 days. Some people might know this but i put it out there for those people who needed help after getting those requirements.