Pokémon Mystery Dungeon: Blue Rescue Team and Red Rescue Team/Mt. Faraway

Grab a key and get in once you have beaten all three fields. It's time to find the Mirage Pokémon. But before you do, you've got to face the final test: Mt. Faraway.

The Dungeon
On the 30th floor, you'll find a locked door. Use the Key to get the Friend Bow.

Also you must have completed Fiery Field, Lighting Field and Northwind Field to gain access. Mt. Faraway has 40 floors(where as the preceding dungeons have only 30), so make sure to bring along enough apples and/or Gummis so your belly doesn't run out on you in the middle of the dungeon. Like the Fields, though traps are abundant and will wear you down if you're particularly unlucky, so it's a good idea to have your leader with the Trap Seer IQ skill(Trap Avoider is also good if you wish to bring along a partner). Other than that, be sure to bring Chesto Berries or a Pokémon with Uproar(Insomnia and Vital Spirit are also helpful) since Parasect will be found on some of the later floors and has an annoying habit of using Spore to knock your entire team to sleep from across the room.

Items
A rare item you can find in here you can't find anywhere else is the Friend Bow(though additional ones can be bought/stolen from Kecleon Shops in Joyous Tower). You find it on the B30F in a locked vault, so you need a key to open it.

Enemies
You'll find an array of Pokémon here. The ones you don't need to worry about are the on around floors 1-20. After level 20 things start getting more difficult.

Boss Battle: Ho-Oh
Finally, the summit. The mirage Pokémon appears. You never suspected it. The High King Ho-Oh. He's not easy (after all, he is the High King). After beating him, return with the friend area Rainbow Peak and defeat him again (he has a recruitment chance of 99.9%).