In the Mario Bros. stage of the one-player mode up to the Master Hand, the Mario Bros. at Peach's Castle are your fourth set of opponents, after the first bonus stage.
You get one partner to help you, and you gang up on Mario and Luigi. Mario should be easy to get, but Luigi will attack you with powerful attacks. You'll need your shield and any items at your disposal for this stage.

There is also a secret stage called the Mushroom Kingdom you access after defeating the Master Hand with the original eight characters and playing on every vs. stage.
The weights from the classic SMB games fall off if one player stands on it too long and the POW from the Mario Bros. game makes all the other players gain 20% damage and they fly high if it's hit while they're on the ground.

As a hidden player, Luigi is only accessable after completing the target stage with the original eight characters.

Mario
A+Up(Smash): Headbutt
This is the perfect attack to use when it looks like an enemy will land on you. Be careful to time it right, though. Also good when an opponent's damage is high for an easy star finish. It has an awful recoil.
Anti: Put a shield on before it hits!!!

A+Up(Normal): Overhead Punch
Great attack to send an opponent flying upwards again and again because it doesn't have as much recoil as the Headbutt.
Anti: Don't fall into your opponent's hands. How hard is that?

A+Side(Smash): Mega Glove(As he punches, Mario's fist gets bigger. I took note o' this and remembered that in Super Mario RPG, Mario's fist gets bigger when he attacks with the Mega Glove. Hence, what I called this attack.)
It has as bad a precoil as the Headbutt's recoil. Time it to knock an opponent clear off near the edge when they're about to make it back from flying far out.
Anti: Make a small and swift move(projectile is best) to stop this attack before it starts.

A+Side(Normal): Mega Kick
Maybe a good attack to use to hit your opponent into a wall again and again.
Anti: Use triple jump to break away from the regular kicking pattern.

A+Down(Smash): Trip(This is another move from another game. Got this one from Super Mario 64.)
Time this right to hit opponents on all sides.
Anti: Put your shield on, quick!

A+Down(Normal): Low Kick
...There isn't really a use for this attack...
Anti: Just roll out o' the way.

A: Punch
A Twice: 2 Punch
A Triple: Kick
A+Dash: Slide Kick(Super Mario 64 again.)
Use this to come up right behind an enemy attacking someone else. >:D
Anti: Be wary o' others sneaking up from behind you.

Mid-air: A: Swift Kick
Simply use this as a vertical Slide Kick.
Anti: Attack back with a more powerful attack.

Mid-air: A+Side: Spiral Kick
More effective than the Swift Kick.
Anti: Same.

Mid-air: A+Down: Drill Kick

While coming down to meet someone, use this to do multiple damage.
Anti: Put your sheild on.

B: Fire Ball
Great attack to stop Samus' or Donkey Kong's charge-up.
Anti: Jump out o' the way.

B+Up: Jump Punch
Use this if you can't get a Drill Kick or Mario Tornado on your opponent.
Anti: Stay agile in the air!

B+Down: Mario Tornado
Come up from below an enemy and get 'em with this. Far better than a Drill Kick.
Anti: Shield.

R: Throw
To the front, it's just a normal throw. But backwards the opponent will knock into and do damage to other foes in the way.
Anti: Before they can grasp you, roll behind them and throw 'em.

Luigi
A+Up(Smash): Headbutt
This is the perfect attack to use when it looks like an enemy will land on you. Be careful to time it right, though. Also good when an opponent's damage is high for an easy star finish. Its precoil isn't as bad as Mario's.
Anti: Put a shield on before it hits!!!

A+Up(Normal): Overhead Punch
It's weak, but use it if Headbutt is too much risk.
Anti: Don't fall into your opponent's hands. How hard is that?

A+Side(Smash): Mega Glove
It has as bad a recoil as the Headbutt's precoil. Time it to knock an opponent clear off near the edge when they're about to make it back from flying far out.
Anti: Make a small and swift move(projectile is best) to stop this attack before it starts.

A+Side(Normal): Mega Kick
Maybe a good attack to use to hit your opponent into a wall again and again. Hard to do with Luigi because o' its power, though.
Anti: Use triple jump to break away from the regular kicking pattern.

A+Down(Smash): Trip(This is another move from another game. Got this one from Super Mario 64.)
Time this right to hit opponents on all sides.
Anti: Put your shield on, quick!

A+Down(Normal): Low Kick
...There isn't really a use for this attack...
Anti: Just roll out o' the way.

A: Punch
A Twice: 2 Punch
A Triple: Kick
A+Dash: Fury Fists
This is great for stopping attacks with precoil.
Anti: Be wary o' others sneaking up from behind you.

Mid-air: A: Swift Kick
Use this for... nothing special. Be careful, Luigi's tallness is his weakness.
Anti: Attack back with a more powerful attack.

Mid-air: A+Side: Spiral Kick
More effective than the Swift Kick.
Anti: Same.

Mid-air: A+Down: Drill Kick

While coming down to meet someone, use this to do multiple damage. This has longer range than ya think.
Anti: Put your sheild on.

B: Fire Ball
Great attack to stop Samus' or Donkey Kong's charge-up. This projectile doesn't last long, though.
Anti: Don't jump into the way.

B+Up: Jump Punch
This is an evil attack. When Luigi's fist is touching them, use it to send them flying.
Anti: Stay agile!

B+Down: Luigi Tornado
Use this to send foes flying with one hit. Not nearly as useful as the Mario Tornado.
Anti: Stay away. This tornado doesn't last for long.

R: Throw
To the front, it's just a normal throw. But backwards the opponent will knock into and do damage to other foes in the way.
Anti: Before they can grasp you, roll behind them and throw 'em.

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