Difficult to say, given how relative in-game battle experience is. I believe I beat Pearl Cynthia on the first try, but I was reviving every other turn in the second half; quite the battle.
Any answer but Mysterial is incorrect. Choice Scarf Kyogre anyone?
For Stadium-styled games I'd probably have to go with this, or Stadium's Final opponents- a legit competitive team plus a Mew and a pure psychic team are just brutal. I pity the Diamond and unlucky Platinum players who had to deal with that. Pearl got off easy.