Era
Dark Ages
The Ocean Palace is a long dungeon filled with a lot of cutscenes. The first area is filled with color-coded Scout type Monsters which are vulnerable to spells of their color. They have high Physical defense, so bring a party that has a diverse range of magic.
This first area has a central room with passages leading off in eight directions. The northeast and northwest passages open bridges leading from the southeast and southwest passages, respectively. Either of these will take you to a large room with another switch that opens the south passage in the first area, which leads to the rest of the Ocean Palace. There is a lot of nice Treasure around as well. Don't miss the chest hidden in the southern room, which can be reached by following the outside path from the western entrance.
Thrashers and Lashers have annoying counterattacks, but they can only use them if there is another enemy of the same kind alive. Try to take them out in one shot with a Combo Tech.
The elevator ride can be a real pain because of the mixed groups of Scouts. Never use the wrong type of magic on a Scout, since they have nasty counterattacks. Also do not attack a Scout if there are others of both colors (unless you kill it), because they have a Triple Tech counterattack. Just take them out one at a time with magic. Mages use Lock, which can make this more difficult.
You can find a Magic Tab by taking the elevator right back up, but it's probably not worth it since you'll have to fight your way back down, starting from the top of the stairs.
When you reach the save point beyond the elevator, be ready for a Boss fight against two Golems. They are basically the same Boss you fought before, only now there are two of them, and you have to win to proceed. Techs that hit both of them are useful to keep them using the same attack type. If you collected the colored vests and/or mail from the sealed chests, you can easily win this fight by equipping them and only using that Element against the Golems. (A Gold Stud, found behind a sealed door in the Future, will help you manage MP.)
Once you've dealt with the Golem Twins, the next Boss is Lavos himself. And not just Lavos, but a super-strong version of him. This isn't even an easy fight in a New Game+, and you're certainly not expected to win your first time through. You'll likely be destroyed very quickly. Enjoy the cutscenes that follow, and get used to not having Crono in the party.
This first area has a central room with passages leading off in eight directions. The northeast and northwest passages open bridges leading from the southeast and southwest passages, respectively. Either of these will take you to a large room with another switch that opens the south passage in the first area, which leads to the rest of the Ocean Palace. There is a lot of nice Treasure around as well. Don't miss the chest hidden in the southern room, which can be reached by following the outside path from the western entrance.
Thrashers and Lashers have annoying counterattacks, but they can only use them if there is another enemy of the same kind alive. Try to take them out in one shot with a Combo Tech.
The elevator ride can be a real pain because of the mixed groups of Scouts. Never use the wrong type of magic on a Scout, since they have nasty counterattacks. Also do not attack a Scout if there are others of both colors (unless you kill it), because they have a Triple Tech counterattack. Just take them out one at a time with magic. Mages use Lock, which can make this more difficult.
You can find a Magic Tab by taking the elevator right back up, but it's probably not worth it since you'll have to fight your way back down, starting from the top of the stairs.
When you reach the save point beyond the elevator, be ready for a Boss fight against two Golems. They are basically the same Boss you fought before, only now there are two of them, and you have to win to proceed. Techs that hit both of them are useful to keep them using the same attack type. If you collected the colored vests and/or mail from the sealed chests, you can easily win this fight by equipping them and only using that Element against the Golems. (A Gold Stud, found behind a sealed door in the Future, will help you manage MP.)
Once you've dealt with the Golem Twins, the next Boss is Lavos himself. And not just Lavos, but a super-strong version of him. This isn't even an easy fight in a New Game+, and you're certainly not expected to win your first time through. You'll likely be destroyed very quickly. Enjoy the cutscenes that follow, and get used to not having Crono in the party.