My hardest battles were a tie between Aya and Sigmund. The main reason Aya was so hard is because I had to not only use my soft reset tactic that I used for almost every important battle in this game (except for Julia, Serra and Noel), but I had to fight her and get ready to go pick up dinner at the same time lol. Despite her bulky ass poison team, I was lucky to have the team I did back then with Infernape, Alakazam, Golduck w/ Zen Headbutt, Donphan, Stunktank and........ Leavanny couldn't do shit lol. Because of them that's the only way that I would have been able to pull off such a risky gameplan involving Psychics, uncertain flinches and crits and BULLDOZE, BULLDOZE, BULLDOZE!!!!
I faced a similar situation with Sigmund, being that he had a stat advantage on me with speed and evasiveness. Cain was wishy washy when it came to cooperation, but that's to be expected with a CPU. But once again, Donphan came in clutch with BULLDOZE, BULLDOZE, BULLDOZE!!!! But he learned Earthquake mid battle so you already know how that goes down. In the end he was no match for me.
Moral of the story: BULLDOZE, BULLDOZE, BULLDOZE Ground types are an essential part of ANY team...