This link will give you all the lowdown on how to beat all the bosses in Final Fantasy II, and there are quite a few of them, too.
This is the first boss you face in the game. He is fairly easy to beat. Have Kain use his Jump attack to cause more damage and Cecil should just plain attack. When he turns into mist, DON'T ATTACK!! If you do, he'll retaliate with ColdBreath, so wait it out until he reverts, then continue the attack.
Octomamm is also a fairly easy boss, so unless you don't know what you're doing, you won't have that much trouble beating him. Have your fighters fight and your magic users cast Lit-1 on him. After a few minutes, he's dust.
This guy is the s.o.b. that's guarding the SandRuby and he's not willing to give it to you without a price. Don't attack him with physical attacks much, 'cause everytime you attack him physically, he counter-attacks with Needle, which is very strong at this point.
MomBomb is actually a group of Bombs and Grey Bombs at first. Kill enough of them and they morph into one gigantic entity. Have your warriors fight and your magic-user cast Ice spells on it. Make sure your character's HPs aren't too low, 'cause when you think you've beaten her, she explodes, causing massive damage.
Milon is the one of Golbez's Four Fiends of Elements who attacks you before Cecil becomes a Paladin. You should have Palom, Porom, and Tellah at this point.Have Palom and Tellah cast their strongest Fire spells and have Porom cure them(when necessary). Just when you think you've beaten him, he morphs intoMilon Z. Once again, have Tellah and Palom cast their strongest Fire spells and he should be history.
The Dark Elf is locateg in the Cave of Magnes located east of Tororia. You'll have to get a Black Chocobo in order to reach it. Inside the cave, you'll have to strip your party down to cloths and non-metallic items. If you have metallic items on while in the cave, that person(s) will be paralyzed when in battle. When you meet the Dark Elf for the first time, you are to purposely lose to him. After that, Edward plays his TwinHarp, which will nullify the spell the Dark Elf placed on the cave. At this point, put your metallic stuff back on and whale away at him.
When you return to Baron Castle, you'll soon find out that Baigan is a monster working for Golbez. You should use caution in fighting him because he casts Wall on himself and if you're not careful, you'll hurt yourself more than you hurt him. Baigan is composed of three parts, himself, his Left Arm and his Right Arm. Take care of the arms first, for they are going to be a bother, then go for his body.
Kainazzo is the second of Golbez's Four Fiends of Elements, who is disguised as the fake King of Baron. Cast your strongest Lit spells, for that will scare him. If you use it enough, he will hide in his shell and will be no bother. Watch out for his wave attack, though. It is quite strong.
You meet these three young ladies at the Tower of Zot. They must be beaten in a certain order unless you want to spend countless days, not to mention hours, fighting them. First go after the short, fat one, which is Cindy. Cindy uses Remedy magic to bring back her defeated siblings, so unless you wish to keep fighting, you'll want to beat her first. Next, go after Mindy, which is the midget-looking one. She uses the annoying Delta Attack, which consists of Fire-2, Ice-2, and Lit-2. And last, fight Sandy, the really tall one. She cast Wall on her friends, so magic is out of the question, unless you have someone in your party that has Wall on him/her, then you can bounce the magic off him/her to attack Sandy. She is the easiest one 'cause she don't attack you.
Valvalis is the third of Golbez's Four Fiends of Elements. You'll find her after you free Kain and Rosa. Have Kain jump to stop her spin. Have your magicians use their spells and your fighters to fight.
You'll meet the wacky doctor and his creation at the top of the Tower of Bab-il. Concentrate on Lugae first because he uses Remedy, then beat up Balnab. Wait a moment, 'cause the Doctor and his creation will become one. Continue fighting them, um, him, er, IT until he's done for.
Don't do anything, just sit there and wait it out. They'll eventually come out of their state and talk to Edge(he should be in your party at this point). They will eventually die on their own.
Rubicant is the leader of the Four Fiends of Elements and is one of the nicer bosses in the game. The reason I say this is because he fully heals you before you fight him, which is good. Don't try fighting him with fire spells 'cause all you going to do is heal him. You can cause major damage with Ice attacks, but you have to wait until he opens his cape, or the ice attacks won't have that much effect. Watch out for his Glance attack; it's very powerful and can cause certain death for some characters. Special Tip: Have Edge cast Flame on him and Rubicant will reveve all the dead members of your party. I told you he is one of the nicer bosses in the game, didn't I?
Fighting her is completely optional, meaning it's not really necessary to fight her. But if you choose to fight her, here's the tip: You'll find her in the Cave of Summoned Monsters. If you have Rosa in your party, she must be above Level 34. She'll be able to cast Wall at this level. Just cast Wall on Asura and keep attacking. What Wall will do is keep her from curing herself, so you'll be able to weaken her. And another thing, she rarely attacks you 'cause she's busy curing. Beat her and Rydia will be able to call her.
You'll also find him in the Cave of Summoned Monsters. Fighing him is also optional. If you do fight him, watch out for his Big Wave attack; powerful stuff. You should cure yourself after he Big Waves you if you are to stand a chance against him. Use Wall to reflect the Ice-2 he casts back at him and use Lightning attacks.
You'll find him in the basement of Baron Castle after you return from the Land of the Summoned Monsters. Like the other guys, he is optional to fight. If you read the books in the library while you were in the Land of the Summoned Monsters, you'll read on something like "Odin was only defeated once when his sword was struck by lightning". So immediately, cast Lit-3(if you have it). It will cause 9,999 HP of damage to him. After casting Lit-3, have your warriors constantly attack him and he'll be subdued in no time. Oh yeah, his sword attack hits for about 4,500 HP, which will instantly kill your party (except if you've fought your way up to the higher levels) so be sure to kill him before he carries out the attack.
These dolls will come to life and attack you when you reach the crystal room behind King Giott's throne. They are actually 3 Cal dolls and 3 Brena dolls. Beat up on the dolls for a while until they form one big doll. Fight the big doll for a while, then the big doll splits up again. Continue fighting until this thing's toast.
This death trap comes after you once you obtained the Crystal from the room. Most of the time you use brute force in fighting him. His petrify attack will turn you to stone if he hits you with it three times. Even though this thing has no weaknesses, attack him with Virus; it works great! His Crush attack instantly kills a person and as far as I know, there is no special defense to avoid it.
Bahamut is located on a cave surrounded by a ring of rocks on the moon. You'll have to face a lot of tough enemies before actually meeting him. When you do meet him, watch for his Mega Nuke attack. If you're not to the right levels, you're just as good as dead when he uses this 'cause this attack can cause up to 4,500 HP of damage to your warriors, so the best way to avoid it: Cast Wall. Cast wall on at least 2 members of your party to reflect the magic back to him. Preferrably you'll want to cast it on your magicians because they can revive any dead characters if you manage to defeat him. Note: He is also one of the Summoned Monsters, so fighting him is also optional.
The CPU is inside the giant robot of Bab-il. It's in the stomach. It consist of three units: The CPU itself, the Attacker and the Defender. The Defender heals the CPU while the Attacker attacks. If the CPU is left alone, meaning if you defeat the Attacker and Defender, it'll use the Globe99 attack, which causes instant death on anybody it's used on. Like the Magus Sisters, they must be defeated in a certain order as well. Preferrably you'll want to take out the Defender first 'cause it constantly heals the CPU, then take out the CPU. When the CPU is defeated, the Attacker will die as well. If you have the Bahamut summon and the Meteo spell, all you have to do is use these two things and call it a day.
This is the final boss of the game. To show his true form, use the crystal Golbez gives you. You should have Cure 4 and the Asura summon by now. If not, prepare yourself for a tough fight. If you collected all the weapons in the core, he shouldn't be that much trouble. His main attack is the Big Bang, which causes up to 2,000 HP of damage on your party. Cure the party afterwards and continue the attack. Repeat this procedure and the king of all evil will be crashing to his doom.