Pokémon Mystery Dungeon: Blue Rescue Team and Red Rescue Team/Lightning Field

The Dungeon
It’s not that hard, but the dungeon is long. Be sure to bring some apples or Gummis to keep your belly full during the trip. Raikou is at floor 30. The only thing to watch out for is Electrode and its Screech.

Items
Thunderstone(s) on the last two floors

Boss Battle: Raikou== Raikou isn't that hard if you have ground type attacks. Also, it has about 650 HP so a simple fly from a level 50 or higher Pokémon can knock him out and recruit him, especially with the friend bow. If you have recruited Groudon, that will be a major advantage, for he is a Ground-type Pokémon. If you are a Flying-type Pokémon, or have the ability Levitate, have Groudon learn Earthquake, so it will only affect Raikou, not you. If your Pokémon is an Electric-type, Raikou won't cause you much damage, but try to get your Pokémon to at least Level 60.