In order to unlock Boss Rush Mode, and a special Bowser X battle, you must beat all of the Gauntlet Bosses in the Challenge Hall.

If you can't seem to suck up a enemy that can be sucked up just repeatably press X and Y instead of just repeatably pressing X. Also when you do it the Power of the move will be doubled.

Here are some Sp. Attack tips if you're stuck!

Mario Bros:

Green Shell: Don't blink. Keep your eye on the shell.
Fire Flower: Don't just mash the A/B button. Tap a steady quaver rhythm.
Jump Helmet: Keep doing the Juuuump Mode.
Yoo Who Cannon: Memorise where one bro. is, then you don't have to worry about the other.
Super Bouncer: Center Luigi if he spins. The spinning will either stop or slow down.
Mighty Meteor: If you're unsure about whether it will hit, don't go full-on at the meteor.
Spin Pipe: Keep your eye on the bro. stacked at the bottom of the pipe.
Snack Basket: It's all about timing. Wait until Luigi gets really high and then slam down with B.
Magic Window: Look for a glimmer of red or green, then you know the bro.
Falling Star: Press both A and B at once as quick as you can.

Bowser:

Goomba Storm: Keep your eye on every Goomba.
Shy Guy Squad: Keep sliding the stylus left until it goes far enough.
Koopa Corps: Go after the shells that are closer to you.
Bob-omb Blitz: Don't aim for the body. Aim for the legs.
Magikoopa Mob: There is no need to trace over the Magikoopas' bodies. Trace the wands only.
Broggy Bonker: If you see a suspicious area, clean it!

We hope these tips work.

go to the chakroad in UNDERGROUND TUNNEL, then go through the green pipe.
go back through the pipe and there will be a pile of wood on the chakroad.
get in front of the block, facing the pipe.pink dots should be coming out.)
press UP quickly NOT REPEATEDLY!!! so that you are ever-so slightly higher on the screen,and so that you cannot see the darkened foreground, then BLOW! (I mean blow fire. <img src="http://i.neoseeker.com/d/icons/bigsmile.gif" border=0 vspace=2 alt="" /> ) now hold RIGHT, until you touch the chakroad. Chakron will ask, "you aspire to travel?"
choose "Don't Travel", then walk around, put do not leave the area. If done correctlythe game's graphics should be all screwey.

Here's how it works: when the chakroad is touched, all of the areas that were not viewed in the space become glitched because they could not load.

To do this glitch, you need to get Mario/Luigi/Bowser's power as high as you can. Now get your character to Lv. 99. As the game wasn't designed for 10000HP+ of damage, if you inflict 10000HP+ of damage, the game immediately freezes.

Have the Mario Bros. go to any enemy in Bowser's body. Then spin jump straight into the enemy. If executed perfectly Mario & Luigi won't say 'Let's-a-go!' until the star you see reaches the touch screen!

For this glitch, you need to be the Mario Bros.. Get out of Bowser's body and go to a place with at least one Toad (Toad Town works best). Now position the bros. so that Mario is facing the Toad, and get as close as you can without changing Mario's action mark to 'speech'. Now Mario must jump over it. If you did it correctly Luigi should walk through the Toad!

After beating Chakron and being launched to Toad Town, you can go back to his location to be launched to Toad Town again.

you know how you can find shroobs in the At the bottom of Bowser's Castle you can find the shroobs from M&L:PiT.

Additionally if you go into fawful theater when you first get there and the fawful show is going on in the theater go all the way to the bottom right. There will be a shroob sitting in the seat if you talk to it it won't say any thing.