Location: World 1-Castle
Good old Bowser returns for this first Boss Battle of the game in a battle reminiscent of the battle at the end of the castle stages in Super Mario Bros.. There are two ways of defeating him. The "proper" way is to use the platform on the left to jump over him and hit the switch, to send him flying into the lava. The other method is to batter him with fireballs until he gives up. Either way, Bowser is defeated, and dies and is cooked in the lava until only his skeleton remains (yes, it's very nasty for a Mario game, but that's what happens!).
 Mega Pokey
Location: World 2-Castle
This one is a bit trickier. Hard Pokey will burrow underground before launching himself out of the sand, firing a few laser shots, then hiding again. The objective is to stomp DIRECTLY ON HIS HEAD - anywhere else will hurt you. It may be preferable to use a normal jump for the first hit and an Aerial Stomp for the second hit, because of his amazing speed in the third round. If you decide to use fireballs, bear in mind that only his head is vulnerable.
 Giga Cheep
Location: World 3-Castle
Giga Cheep is a gigantic Cheep Cheep, who will fly out of the water every few seconds and bounce to attack you, before diving in again. Giga Cheep is accompanied by three Cheep Cheeps, so watch out for them. The basic idea is to jump on or fireball attack Giga Cheep when he is bouncing until he is defeated.
 Giga Goom
Location: World 4-Castle
Not too tricky...Giga Goom is exactly what it says on the tin - a gigantic Goomba. The only way to defeat him without using fireballs is to hit the switch in the centre of the arena, then use the platforms to get up and jump on him. You can also Wall Jump to gain enough height to give his head a stomping.
Location: World 5-Castle
Wahey, everybody's favourite flying Venus fly trap thing is back! Petey will fly around the arena before coming in with a flying kick attack. Avoid him by running BEHIND him as he does this, and jump on his head when he is unconscious after a painful landing. Repeat this twice and Petey is beaten.
 Turret Mole
Location: World 6-Castle
Yes, him again. This time, he's behind the controls of a multi-turret tank. The screen will automatically scroll during this battle. While the shell of the tank cannot harm you, the Bullet Bills which the turrets fire can. When it spins around, duck to avoid its frenzied attacks. It can be attacked in three ways - jumping on Turret Mole's head, launching a fireball at him, or throwing one of his Bob-Ombs back at him. A new turret will appear after each hit. He is considered to be the hardest boss in the game.
 Electro Lakitu
Location: World 7-Castle
Electro Lakitu is a Lakitu who has mastered the power of lightning. Which is very, very bad for you. He will fly around in his cloud, hurling thunderbolts around the arena - run behind him to avoid these. When he flies straight at you, jump on his head or hit him with a fireball or four.
 Undead Bowser
Location: World 8-Castle
This is very similar to the first battle with Bowser (flesh included), except for the fact that there is no platform this time around, AND he is completely immune to fireballs! You can either take a leap of faith over Bowser's head and hope that his horns and bone boomerangs do not hit you to reach the switch, or you can wait until he jumps and run under him to hit the switch.
 Final Boss: Bowser And Bowser Junior
Location: World 8-Final Castle
Yes, Bowser and his kid at the same time. Bowser (now with his flesh regenerated) will launch fireballs at you while Junior uses his standard shell attacks against you. It's probably slightly easier to take out Junior first, using the usual strategy - just watch out for Bowser's fire attacks. After that, Bowser will start hurling ice at you, among other powerful attacks - just throw back whatever you can hit him with. OR, if you feel like taking the easy way out, take a Mega Mushroom with you. Just run into each of them a few times to defeat them, or jump on both of them to easily beat both of them!