With the Masamune in hand, Frog is ready to face off against Magus.
Enter the Castle
Go down both hallways, ignoring the people and getting the chests on the way (left hall, first room: Mid Ether; second room, left: Mid Ether; right: Shelter; right hall: Mid Ether), then head back to the entrance. A fake save point will be there. Once you touch it, monsters appear. Do away with them, and you're ready to move on.
First, take the left hallway. You can avoid the monsters by simply not touching them. You'll want Robo in your party, and it helps if you know the Triple Raid tech. In the room with the throne, talk to the girl up top to fight more monsters.
Then, Slash appears. Have Robo as your third party member. You may want them to know Triple Raid before going into battle.
For the first Slash battle, keep HP above 90. If everyone's in good shape in terms of HP, hit him with Triple Raid. Otherwise, use X Strike.
In the second Slash battle, keep HP above 170 and supplement Robo's healing with Mid Tonics. Only use X Strike. About halfway into the battle, Slash starts using some pretty damaging attacks, including one where he jumps into the air and lands on you with his sword, as well as "Yes, Indeed!". You may have to whip out a Revive or two.
Continue Through the Castle
Get the sword left behind by Slash, save and use a Shelter, then go to the right-hand hallway. Talk to the kids around the chest to fight monsters (one good Slash will finish them off, make sure they're in a position so they all get hit by it) and open the chest (Barrier).
In the next room, don't talk to anyone or you'll have to fight.
Keep Robo in your party, and go to the next room. Make sure Robo and Frog know Bubble Snap.
Fight Flea(?) and Crono should be the one who loses his MP.
Then when the real fight begins, have Robo and Frog use Bubble Snap, and have Crono do all the healing. Keep HP over 100.
Go Further into the Castle
Get the Tab that Flea leaves behind, then go back to the entrance hall. There will be yet another fake save point. Walk into it to be transported to the next part of the castle.
Go up, and keep going up, to fight. When you reach Ozzie, open the chest (Mist Robe), talk to him, and go up.
In the next room, go up the conveyor belts and try to avoid the blades. Get the two chests on the way (first: Dark Mail; second: DoomFinger), then go up.
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, just walk straight down first. You'll fall into a big room and you'll have to fight. Open the chests (left: Mid Ether; mid-left: Lapis; mid-right: Barrier; right: Shelter), get the Tab at the bottom right, but avoid the save points above and below, they're actually monsters. The save point to the left is real, and the one on the right transports you to the room above.
To avoid falling again, first walk all the way left and hug the left wall and go down. Then hug the bottom wall and go right, until you're centered between the walls of the right-hand passage. Go up until your feet are at the same level as the top of the wall to your left, then walk right and hug the right wall and go up. You shouldn't fall.
In the next area, if you see a rolling monster, let it roll off the screen before moving on. Otherwise you'll have to fight it.
For Outlaws, don't attack them until all other monsters on screen are defeated. Jugglers switch between having a magic defense on and a physical defense on. So, for your first attack, use magic, and your second attack should be physical, and you should keep switching up like that until the thing is defeated.
In the next area, you'll have to fight a bunch of monsters. Keep your HP up. When you're done, 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 is a Revive. If you're lucky, you'll even avoid the last fight, which starts as you go up the stairs.
Before fighting Ozzie, get the two chests to the left and right above him (left: Mist Robe; right: MagicScarf).
Don't bother hitting Ozzie. Press left or right to aim at one of the levers nearby, and hit it. Keep hitting the levers until Ozzie's gone.
The right save point is real. 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, you're going to have a lot of trouble up ahead.
Use the left save point to be transported.
Run left to avoid fighting.
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. You really need to heal up when he does this, because he may launch a deadly attack. Keep HP above 300 for this phase. At this phase, attack Magus with your best techs, but leave Robo free to use Heal Beam at every turn.