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

Western Cave
After recruiting Ho-Oh, you'll gain access to Western Cave, home to some of the toughest Pokémon in the game. This includes Mewtwo whose found at the end of this lengthy dungeon.

Surf is needed to enter the dungeon(must be either known by a Pokémon or in the toolbox).

The Dungeon
As a 99 floor dungeon, you'll need to come prepared. Bring plenty of food items(Huge Apples are preferable) as well as Max Elixirs. Oran Berries might not be a bad choice due to how powerful the Pokémon on later floors are.

Items

 * Reviver seed
 * Beauty scarf (key needed on the 59th floor)
 * Keys
 * Poke 10-500
 * Calcium
 * Zinc
 * Iron
 * Protein

Enemies
The Pokémon here are extremely powerful, but as usual, it's the Pokémon on later floors that you want to watch out for. The Tyranitar that appear on floors 50-60 have Sand Stream so they'll whip up a constant sandstorm.

Boss Battle: Mewtwo
Mewtwo waits for you on the 99th floor and while powerful, isn't going to pose the biggest threat to your team, doubly so if you have a Dark type since it negates Confusion. Ghost and Bug types are also strong against him. Sableye, wimp that it is, isn't too bad of a choice being "immune"(read: takes minimal damage from) to both of Mewtwo's only offensive moves.