Birdo-I don't know why people think it's hard to beat her. Just jump onto one of her eggs and pick it up. Then throw it at her. Repeat 3 times.
Mouser-Get ontop of his bombs and pick it up. Before it blows, throw it at him with perfect timing.Repeat this at least 4 times or more I forgot.
Mutaded Crab-Dodge his eggs and(like always) get ontop of it and pick it up. Throw it at him. Repeat like, 6 times I think.
3-headed Snake-Don't grab his fire! Grab the mushroom and throw it at him! A good idea is to hide behind them too while crouching.
Green Birdo(Fire Breathing Birdo)-Try not to go close to her. Find some mushrooms and keep throwing em' at her like 3 times.
King Wart- This idiot won't stop blowing bubbles at you! So wait for a vegie to pop out of the weird trumpet thingy and grab it. To grab it just go let it fall into your hands. With great timing(about 5 seconds) throw it at him while he opens his mouth. But watch out for his bubbles! They'll destroy the vegie. Do this about 6 times.
Here's 2 bonus hints that might help you in some way...
To reset the entire game- During gameplay, press and HOLD the following. Start,Select,A,B.
How to destroy a Shocker-These things are really annoying. If you can find a mushroom, throw it carefully at the shocker.
You can warp from world 1-3 to world 4-1. When you go to world 1-3. Go along like you usually do then when you see a log above two blue shy guys.Pull the bottem left grass. Then take the bottle to the jar past the door.Throw the bottle near the jar. Then go in the door the bottle made.When you go in the door you will see another jar. go in it and you will be warped to world 4-1.