Chrono Trigger/Magus's Castle

Magus's Castle (600 A.D. third time)

Magus's Castle
Incidentally, Ozzie, Slash, and Flea are actually rock musician references, Ozzy Osbourne from Black Sabbath, Slash from Guns N' Roses, and Flea from the Red Hot Chili Peppers, which is why one of the townspeople refers to them as tone-deaf evil fiends.

Anyway, make sure you had Spekkio teach Frog water magic before you begin, and determine who you want your third party member to be. Magus uses barriers of all the different magic types, and Crono and Frog have lightning and water respectively. Marle would be redundant with ice magic, so I'd go with Lucca or Robo for fire or shadow. The beginning of Magus's castle is very eerie, simply follow both paths to their conclusion and come back to the save point in the center to greet Ozzie and begin taking on Magus's henchmen. Head down the left path for Slash, the right path for Flea. I'm going to go with Slash to start since you get a sword for fighting him.

Finding Slash
Take the left path to fight Slash. Talk to the Omnicrones in the next two rooms if you want to fight the skeletons. Get Mid Ether in the first room.

In the last room with the sword on the wall, talk to the lady at the front of the room to fight the skeletons there. After you're finished with them, Slash will confront you.

Afterwards have Crono pick up and equip "Slasher" (but if you charmed Aeon Blade in 65M BC, then don't bother equipping it, Slasher is weaker than Aeon Blade). Also tak Mid Ether and Shelter from 2 chests.

Finding Flea
Backtrack and go down the right path to face Flea. Talk to the bottom kid if you want the Barrier in the treasure chest, these monsters are pretty easy anyway. Keep going through the room and if you want to get into some more monster fights in before the boss, talk to all of the phantoms impersonating people close to your party members and don't forget to get Mid Ether on the way.

You'll then run into a monster who's impersonating the magician Flea. Make sure whatever character kills it has low MP, since whoever finishes it off will have all their MP drained. On a side note, if you counter-attack a monster with a counter, its counter doesn't work so if you have the rage band equipped and it strikes you, you can kill it without getting your MP drained. After you beat the fake flea, the bat that was following you around will turn into the real, androgynous Flea.

After you beat Flea he'll leave behind a Magic Tab.

Finding Ozzie
Head back to the front of the castle and step on the save point to warp forward. You can skirt the edges of the room to avoid most of the monsters or run the gauntlet down the center to fight them. Pick up the Mist Robe at the end and give it to one of your favorite (or active) girls, then talk to Ozzie to chase him away. The next room will be full of conveyor belts and falling anchors, so run as you cross them and be careful. You can also walk against the conveyor belt to stay in place if you need more time to cross. Pick up the chests with Dark Mail and DoomFinger and chase Ozzie into the next room.

The only way to trigger a fight in the next room is by running into the green ball, and previously mentioned you don't get into a fight with monsters unless you're moving yourself. So stay still to let it pass you rather than worrying about climbing the ladders. In case you fight a juggler, it alternates between physical and magical defense. If you hit it with a weapon it'll turn on its physical defense and disable its magical defense and vice versa. It can only have one on at a time, and it starts with physical defense on. In the next room Ozzie will have a bunch of fake platforms that fall out of the floor, leading you to a pit filled with skeletons the first time. I'd recommend falling down at least once to pick up Mid Ether, Lapis, Shelter, Barrier and get the Magic Tab in the bottom right corner. There are four save points, one of which is real, one of which is a transport out, and two of which are monsters called ???s. The fake save points are an excellent source of experience and present no challenge, so fight as many of them as possible. You might want to use a shelter at the save point too since you're probably drained from Slash and Flea.

Once you're through the room you'll pass into another walkway section where either the outlaws or the roly balls can get you into a fight. The next area will have sets of enemies arriving on chains, hopefully Frog has the heal spell by now so you can recover between fights. As noted before, the Jugglers switch between being immune to magic or physical attacks. Try to start off with a multi-target spell and finish them off. The outlaws will do a group counter-attack if you attack them while a juggler is still alive. Get Speed Belt from the chest and go in next room.

The next room is another long hallway with a Revive at the end: you can walk around the statues or charge down the center to fight. Hopefully you have whoever is casting spells to kill the jugglers equipped with the Silver Stud. In the next room you'll finally get to fight Ozzie. Don't worry, it's a joke fight. Pick up the two chests with MagicScarf and Mist Robe, then engage him.

Two save points will appear, the right one of which will actually let you save. Race past some bats after transporting with the left save point, and you'll end up in Magus's sanctum.

Magus
Hopefully Frog has Heal at this point, maybe even Leap Slash. Crono may very well have Lightning 2, and your third party member probably has some powerful fire or shadow magic depending upon whether it's Lucca or Robo. If you want to build tech points, running down the walkway and killing the bats is an easy way to do it. They should fall pretty fast to a multi-attack spell and you can recover at the save point. You may have to use these guys if you can't beat Magus on your first or second try. Pick your party, equip them however you'd like, and get ready to face Magus himself.

When you finally finish him off, Magus will inform you that he only summoned Lavos, he didn't create him. Lavos will begin to wake up and you'll all be sucked into a time gate and disappear.

Challenging Lavos
If you go back and challenge Lavos at this point, you should get The Frog vs Magus Ending. Frog will duel it out with Magus and his henchmen without your help, and the two will finish things off with a battle at the top of the castle.

Also if you lose in the fight to Magus, you'll see a brief scene where he turns around and finishes summoning Lavos, and Lavos will make his trademark scream.