Garland is the easiest boss in the game. He's also known for saying some pretty stupid things. Just make sure your guys are on around level 3 or so and just kick his ass. He'll only hit you with regular attacks and nothing more. I really like garland because he's probably the coolest looking boss IMG.
The king of dark elves, Astos. He's got some nasty moves in store for your party. Reaper, which kills one party member. Slow2, which takes down their speed to practically nothing. He even casts haste on himself. If he busts out fire 2/bolt2/ice2, it'll inflict massive damage upon your party. Plus, he hits a little too hard for my tastes.
If you have the Mythril sword, this battle will be easy. Use Ice 1 on him and have your fighter just pound the crap out of Astos. There's no real strategy to beating him. If he manages to kill one of your characters, restart and try again.
New boss music > joo. Anyway, Lich has some nasty attacks in store for you. His bolt/ice2 stuff deals out the pain. He'll cast sleep and slow on your characters. If he's feeling like he wants to play dirty, he'll just regular attack one of your mages/weaker characters and smear them in a hit. Start off the battle by holding no bars. Use fire3 if you have it, fire 2 works well too. Dia2/3 will bring the pain to him, keep attackers attacking. After while he should go down. Might take awhile since his defence is so goddamn high. If he manages to kill one of your characters (likely) just restart from the memo save you saved in front of him.
Marlilith can hack away your health at a quick rate. Take her out in a few turns. Make sure you have the ice brand on your fighter and cast steel/haste on him. Over 200+ damage a turn should make easy work of her. She can hit pretty hard, Around 200 damage a hit if you don't have good defence. If you're fighting her with nothing more than, say, white mages, then this battle will be very, very hard for you. The only thing that I can tell you do if you're playing with a party like that is hope for luck.
Just hit him a lot with fire 3 and use regular attacks. Dia will hurt him big time. He should go down in one turn. He'll cast rub on you and that really sucks. If it hits, your character will die right there on the spot. Another thing that you should watch out for is that he can hit pretty hard. Either way, this is a super easy battle.
Kraken can hit about 8 hits and clear out one of your people in a hit. His hits do over 400 damage to non-knight characters. Just cast haste on the warrior and use bolt 3 to make quick work of him. To dodge his hits, just cast invis 1 or 2 on your characters. Another thing that he does that's annoying is take forever to die. That's the only reason I recommend haste in this battle, so you can kill him and get through his huge defence. Once he dies, you'll be given the water crystal! Yay.
Tiamat hits hard, but is a total pud. To beat him, simply cast break a lot of times. He'll die within one hit, and you can say that you beat one of the fiends in one hit. If that's something to brag about that it. If you can't get that to work, then caste haste and let at it. He can hit hard with some spells that'll do a lot of damage. His regular attack isn't really all that scary, make sure you take him out quickly before he wears your characters down.
Then the battle starts. He talks about how you killed him. He's Garland, the fiends sent him back. He sent them into the future. He'll be reborn in the past, then you stick a foot in his ass and tell him to shut up.
Cast haste on the warrior. Have him attack, lay flare, holy, cast invis2. Hold nothing back and just brutally assault the minor. He'll cast cure4 sometimes, and restore all of his hit points. His elemental attacks can make one cry since they hit pretty hard.