I made a bunch of these for a similar thread on GameFAQs. Here goes:
Shadow Beasts
Scenario: As long as one lives, they shall never die.
Character: Link (2-stock)
Opponent: Dark-costume Yoshi (x3)
Stage: Bridge of Eldin
Conditions: Defeat the three Yoshis within the 3:00 time limit. Each has only one stock. If two Yoshis are defeated and one remains, the other two will respawn within several seconds. Use those seconds to defeat the last Yoshi. Failure results if there are any Yoshis remaining at the end of the time period or if Link is defeated.
Easy: The Yoshis respawn after five seconds if the third Yoshi is not KOd by that time.
Medium: The Yoshis respawn three seconds if the third Yoshi is not KOd by that time.
Hard: The Yoshis respawn instantly if the third Yoshi is on the field. The match must end with at least a double-KO.
Ice Climbapalooza
Scenario: There's too many of them!
Character: Ice Climbers
Opponent: Ice Climbers (x3)
Stage: Something small; Yoshi's Island for now.
Conditions: All players are gigantic, and Super Mushrooms are set on high. Defeat the other Ice Climbers in a 3-stock match.
Easy: The match is a four-for-all.
Medium: The other Ice Climbers are teamed against you, but friendly fire is on.
Hard: The other Ice Climbers are teamed against you with no friendly fire.
No "Ice" in "Team"
Scenario: Popo and Nana think that they don't need another teammate. Work with the Ice Climbers to take down Bowser.
Character: Select
Opponent: Bowser
Ally: Ice Climbers
Objective: Defeat Bowser in a two-stock match. The Ice Climbers are on your team, but friendly fire is turned on. They will always be up close to Bowser and will only use their standard combo or nAir to attack. Do not KO the Ice Climbers at all while attempting to defeat Bowser. Failure results if the player KOs the Ice Climbers, if the Ice Climbers lose both of their stock, or if the player loses both of his stock.
Easy: Bowser only retreats; he does not attack.
Medium: Bowser will attack.
Hard: The Ice Climbers are not on your team. They will mainly attack Bowser, but will attack you if you get too close.
Upgrade
Conditions: Is it legal to have that many upgrades? You'd better run!
Character: Select (3-stock)
Opponent: Samus (varying stamina)
Stage: Battlefield
Conditions: Survive Samus' attack. Samus is permanently metal, giant, and invisible and possesses a permanent Bunny Hood, Screw Attack, Superspice Curry, and Golden Hammer. She also has a Lip's Stick flower on her head which saps away at her stamina for the entire match.
Easy: Samus has 20 HP
Medium: Samus has 50 HP
Hard: Samus has 100 HP
Faster than Lightning
Scenario: He's dodging your electric attacks! Hit him when he least expects it!
Character: Pikachu
Opponent: Sonic
Stage: Mario Circuit
Conditions: Defeat Sonic in a two-stock match with a time limit. Sonic's AI can read your controller and will automatically defend against any attack involving electricity. Failure results if Pikachu is defeated or if Sonic is still on the field at the end of the time limit.
Easy: 3:00 time limit
Medium: 2:00 time limit
Hard: 1:00 time limit
Crime Doesn't Pay
Scenario: Bowser is on a pilfering spree. Give him a taste of justice!
Character: Select
Opponent: Bowser
Stage: Castle Seige
Conditions: The match is played in Coin Mode. Defeat Bowser before he reaches a certain amount of coins.
Easy: 150 coins
Medium: 100 coins
Hard: 50 coins
No Fair!
Scenario: Charizard feels right at home in this lava-filled chamber!
Character: Select
Opponent: Charizard
Stage: Norfair
Conditions: Defeat Charizard in a 3-stock match. Charizard will not swap to any other pokemon. The magma in Norfair rises far more frequently than in a normal match. Charizard is immune to the rising melt, but your character (even if you play as another Charizard) will take regular damage.
Easy: A magma event activates every 20 seconds.
Medium: A mgama event activates every 10 seconds.
Hard: A magma event activates every 5 seconds.
Purification
Scenario: Link has been corrupted. It is up to Pit to purify him.
Character: Pit
Opponent: Link (Dark Link costume)
Stage: Battlefield
Conditions: Defeat Link in a stamina match. Only Pit's Arrows of Light can damage Link. Failure results if Link is knocked off of the stage or if Pit loses the match.
Easy: Link has 70 stamina
Medium: Link has 150 stamina
Hard: Link has 310 stamina
Eye on the Prize
Scenario: Another trainer wants to surpass you. Prove that you want to be the very best!
Character: Trainer
Ally: Trainer
Opponent: Pikachu, Jigglypuff, Mewtwo (one at a time)
Stage: Pokemon Stadium 2
Conditions: All characters have Stamina rather than Damage percentage. KO at least two of the opponents that appear. The ally trainer will also try to KO the pokemon. Basically, KO more pokemon than the ally trainer.
Easy-Hard: AI trainer's intelligence increases depending on the difficulty setting
Triathlon
Scenario: Are you physically fit? You're your own worst enemy!
Character: Wario
Opponent: Wario
Stage: Custom
Conditions: Beat the computer-controlled Wario in a three-part race. Swim to the first checkpoint. Race on land to the second using your motorcycle. At the second checkpoint, your motorcycle will explode (I know...), and you have to run to the third checkpoint. You can hinder the computer-controlled Wario by attacking.
Protect the Star Rod
Scenario: Bowser seeks to steal the Star Rod... again! Can the King of Dreamland keep his sacred relic from the King of Koopas?
Character: King Dedede
Opponent: Bowser
Stage: Fountain of Dreams, if that is in the game. If not, then just the most relevant Kirby stage.
Conditions: The game is a timed stock match; Dedede has one stock whereas Bowser will always respawn whenever KOd. The only item on the stage is a Star Rod which never fades and never runs out of ammunition. Failure results if Dedede is KOd, if Bowser is holding the Star Rod at the end of the allotted time, or if the Star Rod is destroyed (if it is thrown off the stage or if Bowser is KOd while holding it).
Easy: 1:00 time
Medium: 1:30 time
Hard: 2:00 time
K.K.'s Apprentice
Scenario: K.K. Slider needs a rhythm section. Auditions are Saturdays at 8, so don't blow it.
Character: Donkey Kong
Opponent: None
Stage: Smashville
Conditions: The stage is initially silent. Crush a Smash Ball and activate your Final Smash near Totakeke. At this point, any BGM available in your "My Music" will start to play. Beat your drums at the downbeat of each measure until the smash fades and the stage goes silent once again. Repeat the process four more times. Failure results if you miss too many downbeats, if you somehow stupidly fall off the stage, if you fail to break a Smash Ball before it disappears, or if you audition while Totakeke isn't there (meaning that you have to complete this event when your Wii clock/calendar is set between 20:00 and 21:00 on a Saturday).
Easy: Songs selected will always have slower tempos.
Medium: Songs selected will have a variety of tempos.
Hard: Songs selected will always have faster tempos.
Epic Proportions
Scenario: Wonder what it would be like if characters were actual-size?
Character: Kirby
Opponent: Bowser
Stage: Battlefield
Conditions: Defeat Bowser. Bowser is perpetually enlarged, while Kirby is perpetually shrunken.
Easy: Kirby has 5 stock; Bowser has one stock
Medium: Kirby has 5 stock; Bowser has 2 stock
Hard: Kirby has 3 stock; Bowser has 3 stock
New Microgames!
Scenario: Wario is helping 9-Volt come up with some new microgames. Relive classic Nintendo scenes - but get them right!
Character: Wario
Opponent: Various
Stage: Changes
Conditions: Wario is placed on a stage. Figure out what you're supposed to do, then perform the task. Failure results if you do not figure out/complete the task within three seconds. The stage changes once you complete the task. The microgames are as follows:
1) Battleship Halberd (ship deck) - Meta Knight stands in the corner and a beam sword is at the center of the platform. Take the sword. (Kirby Super Star)
2) Pokemon Stadium 2 (Ground-type stage) - Pikachu stands in front of Wario and breaks Jigglypuff's shield with Headbutt, then runs back to Wario. Wario is holding a pokeball. Throw the Pokeball at Jigglypuff. (Pokemon)
3) Isle Delfino (rooftops) - A Banana, a Freezie, and a Bob-omb are on one side of the stage and Yoshi is shielding on the other side (encasing himself in an egg). Toss the banana to Yoshi. (Super Mario Sunshine)
4) Bridge of Eldin - King Bulbin rides across the stage. Ram into him on your motorcycle. (The Legend of Zelda: Twilight Princess)
5) Battlefield - Wario spawns towards the left edge of the stage. There is a Gordo/Urchin (one of those spiny things seen in the videos) at the edge of the stage, a Home Run Bat near Wario about 1/4 of the way into the stage, an assist trophy at the center of the stage, and a metal box on the left platform. Bowser lunges at Wario. Throw Bowser into the Gordo/Urchin. (Super Mario 64)
Difficulty settings: I need to work this one out...
Need for Speed
Scenario: Sonic wants to proove his worth on the racetrack. Can he outrun F-0's top racer?
Character: Sonic
Opponent: Captain Falcon
Stage: A long, primarily horizontal TSE stage
Conditions: Reach the end of the stage before Captain Falcon. You and C. Falcon cannot damage each other (as though you are on the same team with Friendly Fire turned off). Failure results if Sonic falls off-screen or if Captain Falcon wins the race. (This is really just a timed "reach the finish" as C. Falcon will always reach the goal at an exact amount of time).
Easy: C. Falcon reaches the goal at 1:00
Medium: C. Falcon reaches the goal at 0:50
Hard: C. Falcon reaches the goal at 0:40
Sticky
Scenario: Quick! Get it off before it explodes!
Character: Select
Opponent: Varies by difficulty
Stage: New Pork City
Conditions: Gooey bombs are stuck to all characters aside from one of the computers. These are special gooey bombs which will explode after exactly one minute and insta-KO whoever is caught in the blast. To win, pass your gooey bomb to one of the three speedy characters without a gooey bomb and remain bomb-free at the end of the time limit. All characters are against each other, so every character with a gooey bomb will run for the one character without one. Failure results if a gooey bomb explodes on you, if you fall before the match ends, or if you touch the Ultimate Chimera.
Easy: Opponents are Bowser, Ike, and Dedede
Medium: Opponents are Mario, Link, and Pikachu
Hard: Opponents are Meta Knight, Sonic, and Fox (hehehe...)
Malfunction
Scenario: Zelda's magic is interfering with your Varia Suit. Get her out of there as quickly as possible.
Character: Samus/ZSS
Opponent: Zelda/Shiek
Stage: Whichever new stage plays that awesome OoT Medley
Conditions: Defeat Zelda/Shiek in a three-life stock match with no Smash Balls. Each time that Zelda transforms into Shiek, Samus shifts into ZSS, and every time Shiek transforms back into Zelda, ZSS shifts back into her Varia Suit. Samus has no control over her own transformation.
Easy-Hard: Zelda's intelligence increases depending on the difficulty setting
Armor Toss
Scenario: This armor may have fallen off, but it still has its uses.
Character: ZSS
Opponent: Pit x3
Stage: Skyworld
Conditions: At the start of the battle, Samus automatically uses her Zero Laser to knock three Pits off the stage. However, the Pits respawn one at a time from the edge of the screen with 300% damage and attempt to get back to the stage with their Wings of Palutena ability. The only way to dispose of them is to strike them with remnants of Samus' armor.
Easy: The Pits fly straight at you.
Medium: The Pits fly in easy-to-distinguish patterns.
Hard: The Pits fly in erratic patterns.
Wrecking Crew
Scenario: Who is the master of destruction? Bowser wants to find out!
Character: Bowser
Opponent: Donkey Kong
Stage: Shadow Moses Island
Conditions: The left wall must fall before the right wall. Donkey Kong is working on bringing down the right wall, so it's up to Bowser to stall DK and destroy the left wall before DK finishes his job. The walls have more stamina than they normally would in a regular match.
Easy: Donkey Kong spends more time attacking you than his wall.
Medium: Donkey Kong attacks his wall, but gently compared to Hard mode.
Hard: Donkey Kong seriously wails on that thing.
Buildup
Scenario: Oh-oh! Wario's stomach is starting to bulge!
Charactere: Selelct
Opponent: Wario
Stage: Wario Ware
Conditions: Wario is storing up gas for his Wario Waft throughout the match. KO him before he releases the gas.
Easy: Wario's gas takes twice as long to build up.
Medium: Wario's fills up as usual.
Hard: Wario is halfway full at the start of the match.
Save the Art
Scenario: Diddy's monkeying around is wrecking the whole place!
Character: Ike
Opponent: Diddy
Stage: Castle Siege
Conditions: The battle starts as the stage transitions into Castle Siege's statue room, and the stage will not transition after that. There, Diddy will attempt to destroy the statues. KO him before he can destroy either of the statues.
Easy: Diddy spends more time attacking Ike than the statues.
Medium: Diddy just attacks the statues.
Hard: The statues are already half-broken when the match begins.
Cleanup on Isle Delfino
Scenario: Various characters are taking up residence in Isle Delfino - and they're not welcome! Kick them out as quickly as possible.
Playable character: Select
Opponents: Mario/DK/Yoshi series characters
Stage: Delfino Plaza
Conditions: As usual, the stage will shift from area to area of Delfino Plaza. Each area will have a different opponent. KO each opponent (1 stock) before the stage shifts away from that opponent's area, or you fail.
Easy: Enemies start at 100% damage
Medium: Enemies start at 70% damage
Hard: Enemies start at 50% damage
Return of the SA-X
Scenario: It's back to hunt you, and this time, you're not prepared.
Playable Character: ZSS
Opponent: Samus
Stage: Shadow Moses Island
Conditions: Hide from the SA-X (Samus) for three minutes by using the cloaking devices that appear on the stage one at a time with moderate frequency. Don't get caught by the spotlight, or it will cancel your invisibility and prevent you from using cloaking devices for a short time. Survive until the timer runs out, essentially. No other items, including Smash Balls, spawn during the match.
Easy: Cloaking devices spawn every seven seconds; no spotlights
Medium: Cloaking devices spawn every seven seconds; there are spotlights
Hard: Cloaking devices only spawn when the active cloaking device has three seconds remaining; there are spotlights
Reunion
Scenario: Squirtle has been separated from his trainer. Navigate Squirtle back to Red.
Playable character: Squirtle
Opponent: TSE enemies
Stage: A basic TSE Adventure stage
Conditions: Travel from the start of the adventure stage to Red's location at the end as Squirtle without the ability to switch pokemon.
Easy: No time limit
Medium: 3:00 time limit
Hard: 3:00 time limit with lots of enemies
Sandbag's Revenge, AKA You Must Recover!
Scenario: Sandbag loves getting beaten up, and now he's returning the favor.
Playable character: Select (If you don't plan on using a character with extreme horizontal recovery, however, you're screwed)
Opponent: Sandbag
Stage: Custom
Conditions: Sandbag hits you with the Home Run Bat in the first second of the match, launching you far into the distance. You must safely glide/float/multijump back to the stage, avoiding obstacles in your path. There are some, but few, rest platforms along the way.
Easy: 50m back to the stage
Medium: 100m back to the stage
Hard: 200m back to the stage