Ok read really carefully
Before you enter the pokemon league make sure you have revives and full restore/hyper potions.Yes they are quiet expensive but earning money isn’t that hard.Just go over the pokemon league a few times (you earn more money than loosing them) just don’t use your potions that often.Ok after you defeat the elite four make sure you revive and heal your pokemon before you enter the door to the final room.

PS: those were just hints on getting prepared I’m sure you know what pokemon you should take along.Your pokemon would be at a high level after trying to defeat the elite four.

Yes make sure you have these pokemon types to beat hers- Fire, Grass, Psychic, Water, ice, ghost.

you will be able to beat the actual elite four and cynthia.

catch palkia and teach him fire blast thunder and surf

Add your team with they’re level and held item. And items in bag that you would use and I’ll help you. I’ll keep this on my watchlist so i can edit my answer ok.