Garland awaits you in the Temple of Fiends as the first boss you encounter. You should have your melee characters fully equipped and your magic characters packed with spells. Garland is easy to defeat with a decent party.

Bikke will attack you with 9 pirates when you speak to him in Pravoka. The pirates are very weak, and you do not need any more equipment or magic than you bought from Coneria. Spread out your attacks. After you win, Bikke surrenders his SHIP to your party.

Astos will always cast RUB on one of your allies as his first action. Therefore, it's best to save the game right outside the castle before doing battle so that you can easily re-combat if RUB hits. Astos can hit you with FIRE2 and LIT2, speed himself up with FAST, and slow you down with SLOW2 and SLEEP. It is best to deal swift damage against him and end the battle as soon as possible.

You encounter the wizards right before opening the chest with the CROWN in it. Despite their name, the wizards are totally melee-based, and they're pretty good at it. They do resist most magic, however, but are weak to LIT spells. Focus fire on each wizard individually until all are eliminated.

The Vampire protects the RUBY. He's not a difficult boss, and his only ability is the warrior-stunning DAZZLE. He resists many magics, but is particularly weak to FIRE and HARM spells. Defeat him easily with powerful melee strikes and FIRE/HARM.

Lich is the first of the four fiends you must defeat. He uses every spell in the book, so prepare to heal your allies. AFIR may be considered as a defense from the Lich's NUKE spell. Lich is as magic resistant as the Vampire, but likewise he is weak to FIRE and HARM.

Kary is the second of the four fiends you must defeat. She uses a strong FIRE2, as well as DARK and HOLD. React to her fire attacks with Afir. Kary should be attacked with an ICE-based offensive.

Kraken is the third of the four fiends you must defeat. He uses a strong LIT2, and can blind your warriors with the ability INK (same as DARK). React to his lightning attacks with ALIT. Kraken should be attacked with a LIT-based offensive.

Warmech can be fought at the bridge before Tiamat's chamber. It is the second most powerful enemy in the game, and you are rewarded handsomely for its defeat. React to his special NUCLEAR attack with AFIR, and just hit him with everything you have. Defense comes first, so dish out the defensive spells before total combat.

Tiamat is the last of the four fiends you must defeat. He has multiple special attacks, like THUNDER, POISON GAS, and BLIZZARD. Defend your party with INVS2 and ALIT/AICE. Tiamat resists elemental magic, but try casting BRAK or BANE for an easy win.

Chaos is the last boss of Final Fantasy. He has just about every powerful spell in the game at his disposal, and castings are random. Defend your party with INVS2 and WALL. Chaos resists about all magic, so use your black magic to power your warriors instead. Contrary to popular strategy that involves equipping the MASMUNE to a white mage, I always equip it to a warrior instead. A warrior with that weapon, plus TMPR, FAST, and SABR can do maximum damage to chaos reaching up to 1000. FAST on your Grand Master is a must too. Good luck!