To reach Lavos, you must go to the Ocean Palace.
The Ocean Palace
Use the Gate in the Queen's room in Zeal Palace to go to the Ocean Palace when you are ready. If you need to return to Zeal Palace, you can check on the glowing spot that you arrived on in the Ocean Palace.
The recommended party is Crono, Lucca, and Frog.
Go south and talk to Mune, and watch the cutscene. Use the save point if you wish, then go south and through the door.
Here you will fight Scouters, which are weak to Lightning, Red Scouts, which are weak to Fire, and Blue Scouts, which are weak to Water and Ice.
Go south, then east to reach chest, which contains a Rune Blade, which is a weapon for Frog.
Then use the northeast exit of this room. Step on the switch and two enemies appear. One moves around a lot; defeat it first. When both are gone, go through the southeast exit.
Defeat the Red Scouts and Mage, then open the chest to get an Aeon Helm. Use the west (not northwest) exit.
Use the southwest path and open the chest to get an Aeon Suit. You can avoid the Scouters if you go northeast toward the door that you just came from, then go south from there so you are walking along the narrow path along the wall. Go through the southeast exit of the room.
Go through the south exit, then open the chest to get a Kaiser Arm. Then go back the way you came, through the northwest exit, to return to the large room.
From here, go north, then southwest, then north, and walk west past the column, then go to the northwest exit. Go through.
Step on the switch, and start by defeating the enemy that moves around a lot. When both enemies are gone, go through the southwest exit. Defeat the Mage and Blue Scouts, then open the chest to get a Star Sword, which is a weapon for Crono. Then go through the east (not northeast) exit.
Go south along the edge against the wall, then use the southwest exit. Go west past the Red Scouts and open the chest to get a Shock Wave, which is a weapon for Lucca. Go south from there, and go west and walk along the narrow path against the wall counterclockwise until you see a chest in a small room to the east. You can reach it by walking through the wall. Open the chest to get a Demon Hit, which is a weapon for Frog that does twice as much damage to magical enemies, such as the ones that you encounter here in the Ocean Palace. It is helpful to keep the Demon Hit and not sell it in case you choose to play a New Game +.
Exit the hidden room, then go clockwise around the narrow ledge that you used to reach the hidden room, until you reach the door that goes north. Go south and open the chest in the southeast corner of the platform to get a SonicArrow, which is a weapon for Marle. From there, go east and step on the switch. Go east from there, then north into the room with the two Blue Scouts, then go northwest to return to the large room.
Go northwest, then southwest, then south to the next area.
Go west and down the stairs to reach the next area.
Go south and you will battle some Mages and Thrashers. Go south from there and go down the stairs. You can try to run past the Thrashers to avoid making them jump farther ahead. Continue along the path and battle any Thrashers that you weren't able to avoid.
Continue on the path, and run to avoid getting hit by fireballs, which deal a small amount of damage, and defeat any Thrashers and Lashers that attack. Then continue on the path and there will be another monster encounter. From there, continue going south and there will be a cutscene.
Continue down the stairs to the next area. There is a save point to the west, but you may have to battle the Jinn and Barghest to reach it. Then go south to the next area.
Go south and there will be a monster encounter. Then go to the left side and check on the sparkle. There will be several monster encounters as the elevator goes down. Afterward, go north to the next area.
If you want a Magic Tab as well as more experience, and you don't mind repeating the stairs and elevator, go south and check on the sparkle to take the elevator back up. The Magic Tab is to the left. After you get it, go north through the exit. You have to go down the many stairs again, as well the elevator. Afterward, go north to the next area.
You can go either northwest or northeast. Go through the door and defeat the Scouts and Scouters, and check on the switch that was behind them. Then exit the room and go back to where you first entered this room. Take the other path (northeast if you went northwest, or northwest if you went northeast) and do the same thing.
Leave the room where you flipped the wall switch, and go to the south end of this room. Step on the floor switch and go north across the bridge. Go through the north exit. If you need to go back the way you came, you can go south and step on the floor switch to move the small platform back up.
To the north, there is a chest on the right that contains an Elixir. There is a save point to the left where you can save and use a Shelter.
For the next battle, you will want to use magical elemental attacks that hit multiple targets. It is helpful to bring Robo for his multi-target Heal Beam. It can also be helpful to wear the Mails and Vests that you obtained in Break the Seal. If you are struggling, you may want to go back and use the elevator to earn more experience and skill points.
Go north when you are ready.
The GolemTwins are two Golems, just like the one that you encountered by itself in Zeal Palace. As before, they change their attack mode depending on what they were just attacked with. Since there are two of them, you will be hit twice with their counterattack. It is important to hit them with a multi-target elemental spell as soon as possible, and don't hit them with physical attacks at all during the battle, to prevent them from using their high-damage physical attack.
Dual Techs deal more damage, but you might not yet know multi-target elemental Dual Techs. If this is the case, have Crono use Lightning2 as soon as possible and continue using it throughout the battle, and have your other two party members use a single-target elemental Dual Tech unless the party needs healing.
Each GolemTwin has a Magic Tab that you can steal with Ayla's Charm.
Continue Through the Ocean Palace
Before you move on, go back to the save point, and use a Shelter and save the game. Then go north to the boss room, and north again through the next door, talk to the Nu, then go north.
You'll most likely be defeated, unless you are playing a New Game +. If not, just let it defeat you.
When the time comes, have Crono walk up to Lavos, and watch the cutscenes.
When you're back in control, you can switch party members. Some new Triple Techs will become available.
In the hut, you can check on the straw in the upper-right corner to rest, and you can save at the save point. When you are ready, go outside.
Go into the northern hut, and you can purchase some basic items from the Nu if you wish. Talk to both the Nu and the person in that hut, and then if you leave and go back into that hut and don't move from the spot where you enter, the Nu will exit so you can get the Magic Tab.
Before you proceed, make sure that you are happy with your current selection of party members. You temporarily won't be able to change them.
Go northwest and enter the forested area, known as the Commons. Go north and speak to the Elder, then watch the cutscene. You will end up in a new place.