Here you'll find all the lowdown you need to beat all the bosses in this game.
This is the first boss you meet in the game. He will occasionaly counterattack every time you attack him, resulting in damage for the whole party. He also has a needle attack which causes a good amount of damage. X-Strike and Flame Toss are your best bets at beating him. Stop to heal yourself when necessary.
You'll meet this machine when Crono and Lucca try to escape from the Castle. The Tank is separated into three parts: the head, the main body, and the wheel. The head shoots fireballs at you, causing moderate damage, the Grinder runs you over, doing more damage than the head and body and the tank itself shoots missiles or fires a laser. You'll want to take out the head first, since it repairs any damage to the tank. Next, take out the Grinder to prevent it from counterattacking and last, fight the body.
You'll encounter them when you search for food in the dome. They'll come down from the ceiling and ambush you. You'll notice at the beginning of the battle that the Guardian will begin its countdown. What this means is that it's readying itself for it's big attack, which involves the Bits. To avoid this, take out the Bits first. The Guardian will revive the Bits after time, but don't worry about that. I don't know this for sure, but any attacks that involve magic are useless against it, so you'll have to beat it down with your normal atttacks.
You'll encounter these robots(6 of them) when you go turn on the power in Arris Dome. They jump Robo, which will leave you with only two people. If you're not up to the proper levels, these robots can be a lot more for you to handle. You don't have to worry about their regulat attacks because they are fairly weak, but watch out for their toss attack; this hurts both characters. When there is only one robot left, it uses a bomb attack that causes a lot of damage. Cyclone works pretty against them as it targets three of the robots at one time. Two Cyclones should do them in. As always, heal when necessary.
You'll meet this guy on your way to Belthasar's lab through the sewers. He's pretty easy to beat, just watch out for his multi-slam attack which reduces a character's HP to 1.
Heckran is a really tough monster which you'll find at the end of the cave. He has a wide variety of attacks including a water cyclone attack. Physical attacks will be ineffective, so you'll have to beat him with magic. When he says "Go ahead and try to attack me," don't attack. Attacking him during this phase will cause him to unlease a spell that will do a lot of damage to all of your party. Attack him when the screen says "Brief counterattack break". Also, watch for his attack when he says "Yes Indeed," as he will unleash a multi-hit combo on one of your characters. As for fighting, attack him with your best spells. The double tech "Antipode" should be learned at this point and it causes more damage than the single tech magic. Hit him with a few of these and he'll be toast.
During the attack on Magus's troops in this time period, you meet Ozzie. Ozzie will run away a couple of times then call on Zombor to finish you off. The only attacks you have to watch for is the earthquake attack which will damage all of your characters. Other than that, he's not really hard to beat. The top section is weak against ice and the bottom section is weak against fire, so take advantage of it's weaknesses. When the top section dies, it will cast MP Buster which will take all the MP of the character that killed it. Attack it accordingly and heal if necessary.
You'll find these two on your way to obtain the Masamune. These two aren't nothing more than a test of your worthiness for the famed weapon. They'll do X-Strike against one of your party members every time their turn comes up. At this point, concentrate all your firepower on just one of them. When one of them are defeated, so is the other, but wait! That was only Round 1. Round 2 consists of them joining together into one giant entity. Ice works well against these two as well as the Fire Whirl double tech. Use Crono's Slash technique when it stores tornado energy and this should be an easy battle.
When you finally reach the final room of the Reptite Lair, you'll come up on this big guy who guards the huge Black Tyrano. He's really susceptible to Lightning as it weakens his defenses. Have Crono cast his strongest Lightning attack and constantly whack on Nizbel. After about 2 rounds, Nizbel releases the energy back at your party and boy does it hurt!! You'll want to have a character specialized in healing magic during this battle so he/she can heal your party members after Nizbel's energy charge attacks. Repeat this pattern until this big lizard finally goes down.
This is your final showdown with the evil Reptites and it is a doozy!! Not only do you have to deal with Azala, but also have to watch your guard against the Black Tyrano as well. Your ideal characters should be Crono, Ayla, and someone who can heal your characters whenever needed, preferrably Marle. Start off the battle by attacking Azala until he's gone. Since Azala's a Reptite, he also has the same weaknesses, so use Lightning and the Volt Bite double tech if you have it. Once Azala's out of the way, go against the Black Tyrano. It'll roar, lowering its defenses and a countdown will begin. While it's defenses are down, hit it with your hardest attacks, mainly Volt Bite and Ice Sword. Watch out, once the countdown reaches "0" it'll breathe fire at you, causing major damage and return its defenses to normal. It can also eat one of your party members, causing damage as well. This will be a long battle, but if you keep up the pressure, you'll prevail.
After the battle with Azala and the Black Tyrano, you're automatically sent to fight Lavos. At this stage of the game, you're no match for it. It'll hit you with a meteor shower that causes 3,465 HP to each character, which is too much for your party to handle at the moment. You won't have time to do anything, so just wait it out and watch the scenes afterwards.
This guy is one of Magus's henchmen who you meet inside Magus's Castle. A proficient swordsman, he'll use sword attacks like Slash and a multi-hit combo on your characters. His pattern is pretty much the same for both battles. He's pretty simple to beat; just beat him up and heal yourself when necessary. You'll receive a new weapon for Crono once you've beaten him.
This cross-dresser is another one of Magus's henchmen who you'll run into at the castle. This guy isn't that hard either, but he's pretty annoying because most of his attacks are status affecting attacks, causing one or more of your characters to get confused and/or poisoned. The best way to beat him is to have two characters attack him while the third does most of the healing. Double techs like X-Strike, Blade Toss and Max Cyclone work well against him, so use them until he's toast.
After you've beaten Flea and Slash, you'll soon face Ozzie, Magus's right-hand man. After he flings a few insults at you, he'll cast a barrier on himself which nullifies any attacks coming his way. You'll notice 4 cranks around Ozzie that'll activate trap doors, so whatever you do, don't attack Ozzie. Instead, attack all 4 of the cranks and soon Ozzie will fall to his demise.
At last, the moment of truth. After trudging through the dark castle, you finally reach the lord of the castle. This is going to be a pretty long battle, so make sure you have lots of potions stowed away. Start the battle by attacking Magus with the Masamune, as this will lower his magic defense. Whatever form of magic he uses (ice, lightning, fire, etc.)that's the form of magic you should attack him with. Don't worry if you don't have the magic type he uses to attack you with; just hit him again and he'll change his barrier to the elemental you can attack him with. Around 3/4 of the way through the battle, he'll start using his Dark Matter attack, which causes a superb amount of damage to your party, so make sure you're healed up after he hits you with this otherwise you won't survive. Use Haste to speed things up a little and keep whacking on him until he finally goes down.
Location: 1200 A.D. Zeal Castle
Stats: 7,000 HP
After you encounter Queen Zeal, Dalton, who is one of her followers, leaves a nasty suprise for you in the form of a Golem. It mimics any attack you do; if you use a Fire spell, it'll attack with fire, you use Shadow magic, it uses Shadow and so forth. It can also drop an iron ball on one of your characters, reducing him/her HPs by 1/2. Constantly mix up your attacks and it won't have a chance of retaliating, but watch out for its last-ditch attack when it dies.
Location: 1200 A.D. Mountain of Woe
Stats: (Mud Imp)1,200 HP (Beasts)5,000 HP each.
These trio block your way up to the Mountain of Woe after Zeal kicks you out the castle. The Mud Imp can throw a rock at you that causes one of your party members to be confused and the beasts just attack you physically. Concentrate your firepower on the Mud Imp first 'cause it can heal its friends, then proceed to take down the beasts. The blue one is weak against fire and the red one is weak against ice and water, so capitalize on these weaknesses and you should have them finished off in no time.
Location: 1200 A.D. Mountain of Woe
Stats: (Hands) 2,500 HP each (Head)9,500 HP.
You've finally reached the top of the mountain and you're ready to rescue Belthasar, but before you do, the Giga Gaia shows up to put an end to your crusade. Your best party members at this point are Crono, Ayla, and some other person, preferrably someone who can heal your characters when they're weak. The hands use some powerful attacks; a fire-based double handblaster attack and a power Shadow attack. Use the Falcon Hit double tech to take care of the hands, so that it doesn't attack. Once the hands are out of the way, focus on the head. Oh yeah, the hands are revived every three or four turns, so stay on your guard.
Location: 1200 A.D. Belthasar(a plane)
Stats: 4,500 HP.
After your fiasco with the Giga Gaia, Dalton captures our heroes and traps them on his commandeered plane. During your escape, you encounter this guy and he's not really happy that you want to leave. He's pretty easy, though the only thing to watch out for is his Iron Ball attack, which reduces a character's HP by 1/2, plus he does that last-ditch attack, so as long as you keep your HPs up, there's nothing to worry about.
Location: 1200 A.D. Belthasar Plane
Stats: 15,000 HP.
You'll also find another Golem during your escape from the Wings of Time, but compared to the one you fought earlier, this one's a pushover. Use the same strategy as the Golem you fought earlier on and you'll have no trouble. Midway through the battle, it'll get scared and eventually run away, giving you the victory.