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

Boss Battle: Skarmory
This is the very first boss you have fought yet, and will undoubtedly be the toughest foe you have faced yet. Since his type is Steel/Flying, Cyndaquil, Charmander, and Torchic will have it much easier than the rest. If you picked a Grass type, you had better have a strong Fire type team mate to deal out the damage.

After Skarmory falls, he will fly off, giving you access to the Diglet. After saving it, you will warp back out of the dungeon, and meet the greatly relieved friends.