With the Masamune in hand, Frog is ready to face off against Magus.
Reach the Castle
Magus's Castle can be reached by giving Frog the Masamune and going through the Magic Cave east of Fiona's Villa in 600 A.D.
Enter the Castle
From the entrance, go north up the stairs. The path splits to the west and east. Go west first. Ignore all the people and go north. The chest in the first room contains a Mid Ether. In the second room, the chest on the left contains a Mid Ether, and the chest on the right contains a Shelter.
Go back to where the path splits, and go east. Ignore the children surrounding the chest in the first room, and go north to the second room. Ignore the people here as well. Open the chest to get another Mid Ether. Once you open that chest, you can ignore the rooms to the north and instead go back to the castle entrance.
There will be a fake save point here. To proceed, you have to stand in this fake save point. Monsters will appear. Defeat them, and there will be new monsters in the two hallways.
First, go into the west hallway. You can avoid the monsters by not touching them. Put Robo into your party, and if you haven't learned the Triple Tech Triple Raid, you may wish to fight monsters until you do. Go north to the throne room and talk to the girl standing near the throne, and defeat the monsters that appear.
Afterward, Slash appears. Make Robo your third party member.
In the first battle with Slash, keep your HP above 90. If you don't need to heal, use the Triple Raid tech. Otherwise, use X Strike.
In the second battle with Slash, keep your HP above 170 and have Robo heal, and have Crono and Frog use Mid Tonics as needed. Always attack with X Strike. About halfway into the battle, Slash starts using some more damaging attacks, including one where he jumps into the air and lands on you with his sword, as well as "Yes, Indeed!". Use Revives as needed.
Continue Through the Castle
Go north and check on the sword that Slash left behind to get the Slasher, a weapon for Crono. This save point is real, so save and use a Shelter, then go back to the castle entrance, then go into the eastern path.
Talk to the kids around the chest and say Yes, and you will fight monsters. Crono can use the Slash tech on them to defeat them all at once, but make sure that they are close enough to each other that you will hit them all. Open the chest to get a Barrier, which you can use in battle to reduce magic damage.
Go north to the next room. You can avoid fights by not talking to anyone.
Keep Robo in your party, and go north to the next room. Make sure Robo and Frog know the Dual Tech Bubble Snap.
Talk to Flea(?) to start the battle. Have Crono hit it once with a regular attack, and Crono will be hit with MP Buster and lose all of his MP. This will allow Robo and Frog to keep their MP so they can use magic attacks in the next battle.
Then when the real Flea attacks, have Robo and Frog use Bubble Snap, and have Crono do all the healing. Keep HP over 100.
Go Further into the Castle
Take the Tab that Flea leaves in front of the throne, then go south to the entrance hall. There will be yet another fake save point. You may wish to leave the castle and go outside to use a Shelter and save the game. Then go back to the entrance hall of the castle and walk into the fake save point to be transported to the next part of the castle.
Go north and defeat the monsters that attack. Keep going north until you reach Ozzie. Open the chest (Mist Robe), talk to him, and go north.
In the next room, go across the conveyor belts, but avoid letting the blades fall on you. You will reach a chest that contains a Dark Mail. The next time Ozzie talks to you, he will make the conveyor belts move more quickly. Be sure to hold the B button to run as you avoid the blades. When you reach Ozzie, he runs away. Open the chest near where he was to get a DoomFinger for Robo, then go north up the stairs.
You can avoid fighting here by standing still when a monster rolls by, or by climbing down the ladders and letting the monster roll by.
In the next room, walk south and you will fall through a hole into a large room, where monsters will attack. Afterward, open the chests in the northwest and northeast (left: Mid Ether; mid-left: Lapis; mid-right: Barrier; right: Shelter), then go to the southeast corner and take the Tab there. Don't touch the save points in the north and south, because they are actually monsters. The save point on the west side of the room is real, and the one on the east side of the room transports you to the room above. Use it to return to the previous room.
To avoid falling again, walk up against the west wall and go south, then walk up against the south wall and go east, until you are centered between the walls of the eastern passageway. Go north until your feet are at the same level as the top of the wall on the left, then walk east and stay near the east wall as you go north. If done correctly, you will avoid all of the trap doors. After Ozzie leaves, go north up the stairs.
In this area, if you see a rolling monster, let it roll off of the screen before moving on. Otherwise, you will have to fight it. If you battle Jugglers, be sure to alternate between physical and magical attacks. They start out weak to magic, so use a magical attack first. Then watch for its message saying "physical defense on" or "magical defense on" and use the opposite type of attack from its most recent message. You can also look at them to see if their arms are crossed in front of them (which means it is weak to magic) or if their arms are not crossed (which means it is weak to physical attacks). For Outlaws, don't attack them until all other monsters on screen are defeated.
In the next area, keep going north and defeat all of the monsters that attack. When you reach the north end, after Ozzie leaves, open the chest (Speed Belt).
In the next area, you can avoid fighting by walking behind either one of the statues to the left or right. The chest at the end contains a Revive. Go north up the stairs. If you are lucky, you may be able to avoid the battle that begins as you go north up the stairs, if you are quick enough.
Before you fight Ozzie, open the chests in the northwest and northeast corner (left: Mist Robe; right: MagicScarf). Talk to Ozzie to start the fight.
Don't bother hitting Ozzie. Press left or right to aim at one of the levers nearby, and hit it. Wait for the cutscene, then repeat the process, hitting the levers until Ozzie's gone.
Use the save point on the right to save and use a Shelter. If Robo doesn't know Heal Beam yet, go back and level up until he does. If Frog doesn't know magic yet, things will be challenging. You can go back to the End of Time and talk to Spekkio with Frog in your party to teach him magic.
Use the sparkle on the left to be transported.
Run west to avoid battles. You will reach a dark room.
I recommend Robo as the third member for this battle.
Keep HP above 150 for the first phase of this battle. First, have Crono hit Magus with a physical attack. Magus will change his barrier. If you have the kind of magic that will damage Magus, first have Frog hit him with the Masamune, then use the kind of magic that will damage Magus. Otherwise have Crono do another physical attack to change his barrier again. Repeat the process. Eventually, Magus will stop changing barriers, which marks the beginning of the second phase. Heal up as soon as this happens.
During this phase, keep everyone's HP above 300. Your HP will continually decrease, so keep an eye on it. Attack Magus with your best techs, but leave Robo free to use Heal Beam at every turn.
After the battle, there will be a cutscene, and you will wake up in a different place.