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.

Enemies
Western Cave is 97 floors of pure terror, consisting of Tyranitar, Alakazam, Bayleef and Venasaur. Then one more floor of Charizard, Blastoise and Miltank.

Boss Battle: Mewtwo
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.