To round out the information on Lavos, it should be said that this creature appears to have many forms. We can see at least three of them. This information will be valuable to those who which to slay this evil creature.
The outer shell, which is covered in huge quills (which can be launched at Team Crono with damaging results), is punctuated by a tri-labial orifice that guards some sort of wierd pulsating circular organ. Lavos can copy techniques from other bosses encountered in the game, such as the Dragon Tank and Giga Gaia. On attacking the outer shell's labia-thing, Lavos cycles through all the major bosses, before attacking in a more "generic" manner.
If you don't want to have to fight Lavos' outer shell, and you have the Epoch, you can crash it into the shell and you will drop down into Lavos' innards, which are described below. The Epoch will be destroyed after a reasonably impressive animation sequence, and you will see a different ending as a result.
After you slay the labia-thing, it disappears and a sort of hole (I'll leave the obvious dirty jokes to your imaginations) is left in its place. There are multiple ways to get to Lavos. If you encounter it in 1999, you can simply walk back to a time gate and go back to the End of Time. If you walk through the hole, or crash into Lavos with the Epoch, you will drop into a suspiciously familiar-looking path in its innards, and there is another time gate and a save point there. If you took the shortcut there (the sparkly thing in Lucca's teleporter thingy at the Millenium Fair), there's no going back.
Inner Core #1
As you wander up the dark path inside Lavos, you will begin to hear breathing. As you pass the end of the path, you will enter Lavos' chamber, wherein you will encounter a freaky Gigeresque beast, perhaps fifteen feet tall, with multiple arms and tubing that goes out to the walls of the chamber. It will utter something at you and then unleash hell on you by way of its nipple lasers. (That's not what they are called in the game, but that's what they are!) The head casts magic spells. The arms shoot boomerang-shaped objects at the party. Destroy the left arm as quickly as possible, since it will restore 1,000HP to the head from time to time. It is the weakest element, followed by the right arm and then the body. Luminaire, Ice Sword 2, and Antipode 2 and 3 work well. Magus' spells also work fairly well.
HINT: Lavos' strategy is to use a spell called Active Life to revive parts of itself that you may destroy. It must shut off its own defenses for a short while in order to build up enough energy to carry out the revive portion of the spell. Pay close attention to the "Lavos core shuts off def" and "Lavos core activates def" messages. The right bit is the strongest part of Lavos. Once it dies, you have won.
After the giant Geiger thing dies, a wierd boulliabaise
of what looks like Krang
in a nine-foot-tall armored robotic platypus
body will show up. It will spawn two "bits", which look like floating blobs with tendrils. The bit on the left is susceptible to physical attacks. During the final battle, Lavos will take you into a time warp, through which you will be able to see various scenes from the game. (Some of the best SFX happen here.) Use Crono's Confuse attack and Robo's Uzzi Punch attack to get rid of it as quickly as possible, because it will restore 1,000HP to Lavos' platypus robot body thing, and having three bits means that Lavos can do triple attacks. DO NOT attack it with ANY magic, because this will actually give it back some HP.
After dispatching the left bit, beat the shit out of the body and the right bit with Luminaire, Antipode 3, Dark Mist, or whatever your best magic attacks are. If you have Magus in your crew, you will find it useful to have him cast Magic Wall on everyone. If you have Marle, have her cast Haste on everyone.
Lavos' most dangerous attack is the Dreamless. This will hit your team for some major damage, and it may hit one or two of you with >1000HP (and since you max out at 999, that means that the one or two characters will die). It is a good idea to apply Mega Elixirs judiciously; the moment when Lavos begins to cast Dreamless is probably not the best time to wish that you'd used a Mega Elixir instead of standing around with everyone at 300HP or less.