Pokémon Mystery Dungeon: Blue Rescue Team and Red Rescue Team/Western Cave

After recruiting Ho-Oh, you'll gain access to the game's most terrible dungeon. Western cave is 97 floors of pure terror, consisting of tyranitar, alakazam, bayleef and venasaur. Then one more floor of Charizard, Blastoise and Miltank. On the 99th floor is Mewtwo. After defeating him, you can return and defeat him again. He has a recruitment rate of 99.9%. Congratulations, you have beaten the toughest section of the storyline.