Super Paper Mario/Pit of 100 Trials Flipside

Enemies

 * Goombas
 * Squiglets
 * Koopa Troopas
 * Squigs
 * Sproing-Oings
 * Paragoombas
 * Paratroopas
 * Bald Clefts
 * Boomboxers
 * Jawbus
 * Cherbils
 * Ice Cherbils
 * Spiked Goombas
 * Buzzy Beetles
 * Growmebas
 * Boo
 * Cursyas
 * Magikoopas
 * Bloembas
 * Gloombas
 * Koopa Striker
 * Hammer Bros
 * Poison Cherbils
 * Tileoid Gs
 * Tileoid Bs
 * Crazee Dayzees
 * Chain Chomp
 * Spinies
 * Longators
 * Choppa
 * Boing-Oing
 * Beepboxers
 * Hooligon
 * Fuzzy
 * Tileoid Rs
 * Pigarithm
 * Tileoid Ys
 * Moon Cleft
 * Shlorp
 * Pokey
 * Squogs
 * Clubbas
 * Rawbuses
 * Tech Cursyas
 * Amazy Dayzee
 * Spike Top
 * Spanias
 * Heavy Cursyas
 * Reversya Cursyas
 * Ninjoes
 * Dry Bones (1)
 * Dark Boos
 * Dull Bones (1)
 * Ruff Puff
 * Skellobits
 * Spiky Skellobits
 * Boomerang Bros
 * Koopatrols
 * Blue Magiblots
 * Soopa Striker
 * Fire Bros
 * Gawbus
 * Red Magiblots
 * Zoing-Oing
 * Blastboxer
 * Chroembas
 * Longadile
 * Yellow Magiblots
 * Red Chomp
 * Poison Pokey
 * Pink Fuzzy
 * Ninjohns
 * Ninjerrys
 * Squoinkers
 * Headbonk Goomba
 * Wracktail (Flipside Boss)
 * Wrackle (Fought Together With Wracktail)

Check the floor you want to reach and see if you have met the requirements.

If you have all that, if you have Luigi go to Flipside B2, use super jump and hit the blue switch.

If you don't have Luigi, go to Flipside Outskirts, go down the pipe to your left, Go down another pipe to your left, find the hidden blocks with Tippi, jump across the blocks and hit the switch.

Each room in the Pit of 100 Trials resembles the LCD display from Nintendo's old Game & Watch devices. It is highly recommended to stay in 2D mode for this reason because flipping to 3D reveals the 2D nature of the backdrop, causing you to fall to the floor.

In every room there are a few enemies. One random enemy has the key. Kill the enemies until one drops a key. When you get the key, unlock and go through the door. Remember, each room has a time limit, so work quickly. Every tenth room you get a card as well as the opportunity to bail out of the pit and try again from the beginning another time.