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 fl.30. The only thing to watch out for is electrode and its screech.

Items
Thunderstone

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.