Whether you level up or not, the game is still easy.
Also the card game is incredibly boring, so I wouldn't hae the patience to get the cards you mentioned. I'd rather just level incredibly high.
I read somewhere that if you're level 99 or 100 Ultimecia gets another 50k hp, but when your Renzokuken does 9999 every hit, thats just one more limit you have to use.
I thought when you had the RSP right you don't fight any monsters down there. I may be wrong as I haven't played the game in like forever.