? HP Attack: 1 Defense: 0
This is a story-battle. You can't win it. Just keep attacking until it ends.
Jr. Troopa(1)
5 HP Attack: 1 Defense: 0
There's no trick to winning this battle. All you'll have to do is trade hits with Jr. Troopa, and you'll come out victorious.
Blue Goomba and Red Goomba
6 HP (Blue)

7 HP (Red)

Attack: 1 Defense: 0
Have Mario use his Power Jump on Blue Goomba, and have Goombario attack Red Goomba. After two Power Jumps, Blue Goomba should be defeated. Then have Mario and Goombario attack Red Goomba until he's also defeated.
Goomba King, Blue Goomba and Red Goomba
2 HP (Blue and Red)

10 HP (King)

Attack: 1 Defense: 0
At the beginning of the battle, attack the Goomnut Tree. After you attack it, a spiky goomnut will drop down, and defeat Red and Blue Goomba, and damage the Goomba King. After this, just use your most powerful attacks on the Goomba King.
8 HP Attack: 3 Defense: 0
The Magikoopa is basically just to get you used to the Action Command. Just attack him and use your Action Command and you shouldn't have any problem.
4 Fuzzies
3 HP Attack: 1 Defense: 0
Have Kooper use his Power Shell attack to damage all four fuzzies. Then just attack normally until they're all defeated.
3 Bill Blasters
4 HP Attack: 0 Defense: 1
The Bill Blasters have relatively high defense, so you'll probably have to use Mario's Hammer to damage them. Goombario also won't be able to damage them. So, use hammer attacks and Bombette or Kooper to attack the Blasters, and when they summon Bullet Bills, you may also wish to attack them, as they will attack you the next round if you don't defeat them.
10 HP Attack: 1 Defense: 1
Attack using Mario's Hammer and Bombette or Kooper. This battle's not too tough, and be sure to save your FP.
Koopa Bros.
? HP

5 HP (alone)

Attack: ?

1 (alone)

Defense: ?

1 (alone)

Have Bombette start out by using her Bomb. This will knock the tower over. While the Ninjakoopas are separated, treat them like normal Koopa Troopas. If the tower reforms, have Bombette bomb it again, or Mario and his partner each attack it, and they'll topple over again.
Jr. Troopa(2)
15 HP Attack: 2 Defense: 1
Jr. Troopa has added a little bit of defense since last time, so use your hammer on him. Bombette's your most powerful ally right now, so her and her Bomb ability are your best bets for this battle.
30 HP Attack: 3 Defense: 0
Blooper has 30 HP, but he's not too hard.
15 HP Attack: 2 Defense: 2
Chan is basically a souped-up Buzzy Beetle. He's also very easy to defeat. Just use Jump attacks, which will flip him over, and you shouldn't have any problem.
40 HP Attack: 3 Defense: 0
You don't have to fight Buzzar, you can answer "Luigi" to his question, and he will let you go. If you do choose to fight him, the most difficult thing about him is his high HP. Parakarry's Shell Shot is useful here, as is Mario's Power Bounce.
30 HP Attack: 3 Defense: 0
Tutankoopa begins the fight with a Chomp. It's up to you whether or not you want to defeat the Chomp, but I usually leave it, since it disappears once you defeat Tutankoopa. Just use Mario's powerful jumping attacks and Parakarry and you should be fine.
Electro Blooper
50 HP Attack: 4 Defense: 0
The Electro Blooper attacks much in the same way as the Blooper you fought earlier, but he has the ability to electrify himself, which will raise his attack power of his next attack. You can defuse his electricity by attacking him, although attacking him is difficult when he's electrified, since you can't use your hammer since he's floating, and you can't jump on him, since he's electric (unless you're electrified yourself from the Zap Tap Badge or a Volt Shroom). Mario's Throw Hammer attack and Parakarry's Shell Shot are effective at de-electrifying him, though. When he's not electric, have Mario use Power Bounce.
20 HP Attack: 5 Defense: 0
Lee isn't much harder than Chan. He will imitate Mario's partner, so use this to your advantage. Enter the fight with Kooper, and when he transforms into Kooper, you can flip him over just like a regular Koopa Troopa. Then he's easy to defeat.
Tubba's Heart
50 HP Attack: 6 Defense: 0
Tubba's Heart is an extremely easy battle IF you have Bow as your partner. Simply jump on him/slap him until he charges himself up, then have Bow use Outta Sight. Repeat this pattern until you're victorious.
Tubba Blubba
10 HP Attack: 4 Defense: 0
When Tubba Blubba reunites with his heart, he actually becomes much easier. He only 10 HP, so you should beat him in a round or two. The only thing you have to worry about is if you have extremely low HP from the battle with his heart. If not, this battle is cake.
Jr. Troopa(3)
40 HP Attack: 5 Defense: 1
Jr. Troopa's new-found ability is flying, so you won't be able to use your hammer. Jumping on him still works wonders though. Have Mario use his most powerful jump attacks, and Parakarry use his Shell Shot. You may want to try out Star Storm as well, for a nice 7 HP attack.
The Master(1)
50 HP Attack: 6 Defense: 0
There's no real trick to this battle, so if you're having trouble, you may want to just come back later in the game. Anyway, Bow is a good choice for a partner, since The Master has low defense.
Anti Guy
50 HP Attack: 10 Defense: 0
Fighting the Anti Guy is completely optional, and he's very difficult (especially for the point in the game you first encounter him), so you may want to refrain from fighting him until later in the game. You co want to fight him eventually, since he's guarding a Power Plus Badge. Or you can come back and bribe him with a Lemon Candy, which will have him leave without a fight. If you do want to fight him, Bow is a good choice for a partner, since she can hide Mario, and her slap attacks cause good damage. Oh, and be sure to have plenty of healing items on hand, as well as the Double Dip Badge.
Big Lantern Ghost
40 HP Attack: 5 Defense: 0
If you know what you're doing, this is a pretty easy fight. You can't attack the ghost unless the lantern is lit. To light the lantern, you have to attack it twice. So bring a partner who can attack twice in one turn (Goombario works, but it's much easier with Bow). Have the partner attack first, and light the room, then have Mario use a powerful attack on the Big Lantern Ghost. When the lantern goes out again, repeat the process.
General Guy
30 HP Attack: 4 Defense: 2
Before fighting General Guy, you'll have to first fight the Shy Squad, then two Stilt Guys, then two Shy Stacks. The Shy Squad's attack power is dependent on the number of Shy Guys present (its HP), so it's helpful to defeat them quickly using powerful attacks. The Stilt Guys can fall off their stilts and become regular Shy Guys if you use your action command with a jump attack or a quake attack. The Shy Stacks can lose members if Mario uses his "Spin Smash" ability, but it's easier just to attack them. When you finally fight General Guy, the first thing you'll want to do is attack its lightbulb with Mario and your partner (who should be Watt). When the lightbulb is gone, have Mario and Watt attack the tank. Since the tank has high defense, Mario should use his hammer attacks, and Watt's Electro Dash ignores defense, so the defense isn't a factor for her.
The Master(2)
75 HP Attack: 8 Defense: 0
The second battle with The Master is much more difficult than the first, especially since The Master has a combo attack that attacks twice for heavy damage. The best way to defend against this is by being electrified, preferably with the Zap Tap Badge, but a good supply of Volt Shrooms will work too. Bow is a good choice for a partner, as she can cause good damage, and hide Mario when the going gets rough.
20 HP Attack: 3 Defense: 0
The only thing you have to remember about Fuzzipede is that Watt MUST be your partner, or you won't be able to attack him. He only has 20 HP, so this should be a quick fight.
Lava Piranha(1)
40 HP Attack: 5 Defense: 0
Be sure to have the Fire Shield and Ice Power Badges equipped for this fight. It's nice to start the battle off with something that will attack all enemies, like the Star Storm. Then just attack each of the buds to take them out of commission. No concentrate your attacks on the Lava Piranha itself, and you should emerge victorious...however...
Lava Piranha(2)
40 HP Attack: 5 Defense: 0
...The Lava Piranha will return immediately, and be on fire this time. This means that you won't be able to jump on it unless the Ice Power Badge is equipped. This also means that he's now weak against water and ice attacks. In addition, water/ice attacks will temporarily stun him. If Sushie is at Ultra-Rank, then the Tidal Wave is immensely useful. If not, her Squirt attack is still very helpful. Snowman Dolls come in handy as well. Anyway, use whatever water/ice attacks you have in your arsenal to stun the plant, then attack regularly until he inflames himself again. If the Buds summon Petit Piranhas, it will probably be worth your while to defeat them before they have a chance to attack.
Jr. Troopa(4)
20 HP Attack: 6 Defense: 0
Although Jr. Troopa is now immune to both Hammer and Jump attacks (Unless you have the Spike Shield Badge equipped), this is still an extremely easy battle. Have Mario use Star Storm every chance he gets, and have Parakarry use Shell Shot. He should be defeated within two rounds.
Kent C. Koopa
70 HP Attack: 10 Defense: 6
Kent C. Koopa blocks Pleasant Path after Chapter 5. He can be very easy or very difficult, depending on how you go about fighting him. He has sickeningly high defense, so Watt is useful here, and he can be knocked on his back, but you'll need a double jump to do it. When he's on his back, use your most powerful attacks. Or you can just put him to sleep with a Sleepy Sheep or the Lullaby Star Power. While he's asleep, he can't do anything, and he becomes extremely easy to defeat.
50 HP Attack: 4 Defense: 0
This is basically a lakitu with slightly higher attack power and much more HP than a regular Lakitu, so he's not very difficult. Just use Mario's powerful jump attacks, and the partner of your choice, and you should have no problem.
Huff N. Puff
60 HP Attack: 5 Defense: 0
The problem with Huff N. Puff is that for every HP damage you do, he will spawn a Tuff Puff. He can swallow these Tuff Puffs to restore his energy. So basically, you'll want to attack him heavily, then defeat the Tuff Puffs before he can swallow them. So have Mario attack, then have Lakilester use Spiny Surge to attack and defeat any Tuff Puffs left over from Mario's attack. You'll want to do this every round, so he won't be able to restore much HP. When he gets angry, it may be worthwhile to switch to Bow and use Outta Sight, it's your call. Lakilester's Cloud Nine is helpful too, if you have it.
Super Blooper
70 HP Attack: 5 Defense: 0
If you have the Zap Tap Badge, be sure to equip for this battle. This way, when Blooper Babies are summoned, they won't be able to hurt you, and you won't have to waste turns by attacking them, either. Volt Shrooms will have to do if you don't have the Zap Tap Badge yet. Bow is a great choice for a partner, especially if you have the Fan Smack. Also, her Outta Sight is invaluable for when Super Blooper powers himself up.
Jr. Troopa(5)
50 HP Attack: 8 Defense: 1
Jr. Troopa has forsaken his spike and wings for the ability to cast magic. He has a high attack power, but other than that, he's still pretty easy. Just keep chugging away with your most powerful attacks, and heal using Smooch if you get in trouble.
20 HP Attack: 1 Defense: 0
This is more of a storyline battle than anything, so it's supposed to be really easy. Just keep attacking until you win.
Crystal King
70 HP Attack: 6 Defense: 2
The difficulty with Crystal King stems from his summoning of Crystal Bits. I would suggest killing two of the Crystal Bits when he summons them. That way, when he spits them out, it will only cause you a maximum of 4 HP damage. When attacking the Crystal King himself, Watt is a good choice, since Electro Shock ignores defense, and costs no FP. When he is weakened, he has the ability to restore 20 HP (ouch), and split himself into three parts. When he's split, only one of his parts will take damage, so the safest bet is to use an attack that damages all enemies, like Lakilester's Spiny Surge. You're in for a long fight, so bring along some healing items.
The Master(3)
99 HP Attack: 10 Defense: 1
The toughest incarnation of The Master. The Zap Tap Badge is immensely valuable here, since it will interrupt The Master's combo attack, as well as damage him. There's no clear cut choice for a best partner here, but I like to use Watt, since she doesn't use any FP with her Electro Shock, which saves the FP for Mario's powerful attacks. The Master has lots of HP, but with proper healing and the Zap Tap Badge, you should be able to handle him.
3 Anti Guys
50 HP Attack: 10 Defense: 0
If you get three wrong answers on the second Bowser Door in Bowser's Castle, you'll have to fight three Anti Guys at once. If you want a challenge, get the wrong answers on purpose and fight the Anti Guys. It is essential to use your most powerful attacks to defeat each Anti Guy, one at a time, as quickly as possible, since the Anti Guys' attack power is so high. Bow is an excellent choice, since her Fan Smack can do up to 12 HP damage, plus you may need to use Outta Sight to protect Mario and give him a chance to heal if the going gets too rough. Be sure your inventory is full of powerful healing items, and you have the Double Dip or Triple Dip Badge equipped. The Defense Plus Badge, Zap Tap Badge, and Attack Plus Badges also are helpful.
Jr. Troopa(6)
60 HP Attack: 8 Defense: 2
This is the last time you'll have to fight Jr. Troopa, and also the hardest. He will start out in his normal form, but then sprout his wings and spike, and then regain his magic rod. Two things will make this battle easier: the Spike Shield Badge and Watt as a partner. Watt is able to cut through his high defense, and the Spike Shield will enable you to jump on him while he's flying. He can heal himself for 10 HP, but he's usually defeated before he has the chance.
50 HP Attack: 6 or 8 Defense: 1
This battle with Bowser is really pretty easy. You'll want Watt as your partner, since she will be able attack Bowser for a consistent 5 HP, without using any FP. Have Mario use his most powerful attacks, and whenever Bowser powers himself up, use the Star Beam immediately.
99 HP Attack: 8 or 10 Defense: 2
This is a story-battle that you can't win. Just fight with Bowser until he powers himself up, then use the Star Beam. When you realize that it doesn't work, the battle will end automatically.
Kammy Koopa
? HP Attack: ? Defense: ?
The coolest thing about this fight is that it's Twink and Peach fighting against Kammy. This is another story-battle, so just follow the prompts and you'll win. You can't lose even if you try.
99 HP Attack: 8 or 10 Defense: 2
This is it, the final battle. First of all, you'll want Watt to be your partner. Each attack she does will be an efficient 5 HP, and it won't cost you any FP. Second of all, make sure you have all your healing items available (and some Life Shrooms). Since this is the last battle, don't be shy about using all your best items, like Ultra Shrooms and Jammin' Jellys. The Double/Triple Dip Badge wouldn't hurt either, in case you get in serious trouble. Use your absolute most powerful attacks with Mario, and Electro Dash with Watt. Bowser can heal himself for a whopping 30 HP, and will do so several times during this battle, so this will take a while, but it's not as bad as it seems.