All roads lead to Lavos... but there are many ways to get there. Passing through the Black Dream is one. Taking the bucket Gate from the Farthest Reaches of Time is another. Flying to 1999A.D. in the Sylbird is a third, and bypasses fighting the shell. And there's also a fourth way, but that's special....

In any case, I only have going through the Black Dream covered at the moment (hopefully to change soonish eventually.).

The party finds themselves facing Lavos in a distorted dimension. Lavos's Theme plays, of course.

The image of the Dragon Tank boss from early in the game appears briefly. Lavos now has two side... somethings... that act like the tank's wheel and head, and Lavos itself emulates the tank body. After taking a pitiful amount of damage....

This time the image shows the Guard Machine from outside the food storage room in the future. Lavos keeps changing attack mode every time one is beaten, going through Hekeran, Junkdragar, Gran and Leon, Nizbel, Magus (such nerve!), Azarla and the Black Tyrano, and Giga Gaia. The party gets a chance to regroup and heal between each battle. After the last emulated boss, Lavos opens its craw (?), makes the Evil Lavos Noise, and attacks as itself. However, it really isn't all that difficult... almost too easy, really, especially compared to fighting it at the Abyssal Temple. Beating the head (?) reveals a gaping hole in the shell....

Marle: W, we did it...!

Marle: Unbelievable...!

Lucca: It, it's over....

Lucca: It's finally over...

Robo: W, we DID it!!

Robo: We were successful!!

Frog: We did it!?

Frog: Can it be over?!

Ayla: Beat Lavos!
Aylas strongest!!

Ayla: Lavos down!
Ayla No1!!

Magus: We did it...?

Magus: It's over?

Marle: But it doesn't seem like just
something like this would finish it...!

Marle: But THIS couldn't have done
him in...!

Lucca: I, I wish it would just go
ahead and end now....

Lucca: I wish it were over...

Robo: From inside Lavos's body,
a still more powerful life energy
IS detected...!

Robo: I sense a powerful life force
within Lavos.

Frog: But it doesn't seem like it's
over just yet....

Frog: Will there never be an
end...?

Ayla: This Lavos's shell!
Insides still healthy!!

Ayla: This Lavos's shell!
Inside still strong!!

Magus: Don't celebrate!
It's still too early to be relax...!

Magus: It ain't over yet...

Marle: L, let's try going inside!

Marle: L...let's go inside!

Lucca: There's no choice but to go in.
I was afraid of this....

Lucca: I guess we have no choice.
But...

Robo: We must INVADE the
interior and defeat the CORE part!

Robo: We must invade the interior
and attack its...core!

Frog: This's just armor, in the end.
Got to beat the insides to death...!

Frog: This be only armor!
'Tis what's inside that must be
destroyed!

Ayla: Ayla not lose!!

Ayla: Ayla no give up!!

Magus: I'll put an end to your life.
Lavos...!

Magus: We'll get to the bottom of
this, Lavos...

??? is the actual location name listed on games saved here.

The party enters the hole and drops into a cave deep inside.

Marle: We've come awfully
deep inside....

Marle: We're in pretty deep...

Lucca: W, we're okay, right?

Lucca: We're okay...right?

Robo: According to my sensor
readings, the core IS close!

Robo: According to my sensors,
we are close to the center!

Frog: Creepy place.
More disturbing than Magus Castle....

Frog: This is creepy.
It's worse than Magus's place...

Ayla: !!
Lavos's smell get stronger!!

Ayla: !!
Lavos smell...STRONGER!!

Magus: ...can't back out any more.

Magus: ...No turning back now.

If you got here through the Black Dream or the bucket at the Farthest Reaches of Time, there are both a save point and a Gate leading back to the Farthest Reaches of Time (otherwise, they aren't here). In fact, it's not a bad idea to head through the Gate now to heal up and restock supplies. The shell stays beaten and the hole open, so there's no reason not to.

Old Man: The Black Dream is gone, and that
woman was at long last able to take her rest.
She was able to return to being human....
Now is the time to dive into the Gate from
the bucket over there...!
This is not just your battle.
The lives on this planet....
Indeed, the battle is between this planet
and Lavos...!

OLD MAN: That poor woman can finally
rest, now that the Black Omen is
gone.
She's reverted back to a human...
Now it is time for you to access the
Gate contained within that bucket!
You fight not just for yourselves, but
for all living things...
This is a battle between Lavos and
the entire world!

As the party moves farther along the tunnel, the sounds of breathing and a heartbeat grow stronger.... At the end of a tunnel is what looks like a humanoid armored creature with huge cables attached (energy? data? life support?). 3-13 世界変革の時 (When the World Reforms) plays.

I don't usually comment on the track names, but this one deserves it. To translate and paraphrase the definition from goo's dictionary function, 変革 (henkaku) means "changing and making new", so the battle is to change and renew the world.

Marle: This... is Lavos!

Marle: This...is...Lavos!

Lucca: Th, this... is it's true form!?

Lucca: This's the REAL thing?!

Robo: I! I can NOT believe it...!
It HAS the genes of all lifeforms
since life was born on this planet!

Robo: Amazing...
It houses all the DNA of every
creature that ever lived...!

Frog: Fight by my side...
Cyrus!

Frog: Cyrus, this be our finest
battle...

Ayla: Ayla not lose!!

Ayla: Ayla not give up!!

Magus: This time, for certain...
It will be your last, Lavos...!

Magus: Finally...
You have met your match, Lavos!...

This stage of Lavos has a main body and two arms. The main body starts out inactive and nearly invulnerable. The arms use a variety of moves, including:

Lasers blast the entire party, inflicting magic damage. As the description suggests, this attack is only usable when both arms are present.

Negates one character's protection against status effects. This attack only seems to occur when one arm is left, so try killing both at once if possible.

After both arms die or enough time passes, the body becomes active (and vulnerable) and cycles through the following five abilities until dying:

Confuses any character who lacks protection, and deals some damage to all.

邪気 doesn't translate well, but means something similar to "evil aura" or "malice". Jyaki's name is also a homophone of this.

Poisons any character who lacks protection, and deals some damage to all.

Fire damage to one character, insignificant with high magic defense.

Non-elemental magic damage to all characters.

Interesting that the name for this attack is just the names from the last three shown all at once. And yes, I did choose the words in the other names carefully so that the effect would work in English, but it was mostly a matter of picking appropriate synonyms and parts of speech. It's still not perfect—if I were to translate the final attack on its own, it would come out more like "evil shadow flames that kill your will to fight"—but it works fairly well.

When the body dies, the music fades. The figure the party was fighting is replaced by a smaller vaguely humanoid figure wearing what looks like a space suit.

Marle: H, human form....
It, it, don't tell me....
It couldn't be armed with the powers
of every living thing on this planet...?

Marle: It's...humanoid...
It seems like it has collected all of
the vitality from the creatures who
have lived on this planet.

The following is unused. It looks like there's a "wait, then end text block" code where there should be a "wait, then clear text and continue".

Lucca: I, I've figured it out.
What it's after....
It's parasitic on a planet, and picks out
and collects the superior parts of the life
forms on that planet over a long time....
It bears children with those genes there on
Death Mountain, then on to another planet....

Lucca: Now I understand...
It lives on a planet for as long as
possible, stealing away the most vital
resources...
It combined the DNA it found here
with its own, and gave birth to those
creatures up on Death Peak.
Eventually the young must migrate to
other planets...to repeat the cycle...

Robo: TH, THIS was Lavos's goal...!
TO gain a record of the genes of
every lifeform on the planet....
And TO further its own evolution!

Robo: This was Lavos's goal...!
Using the DNA of every organism...
And achieving the ultimate in
evolution!

Frog: Wh, what the hell's going on...!?
Wait, don't tell me it's got the powers
of every living thing on this planet....
That's it! Us, everything that's evolved
in every living thing on this planet....
The lowlife's been hoarding it all
while it sleeps...!!

Frog: This be evil!
Indeed! This thing possesseth the
vitality of all living creatures...
It hath harvested DNA from animals,
only to further its own evolution!
And whilst sleeping, to boot!

What are the odds Frog has any idea what DNA even is? Not good.

Ayla: This Aylas!
Dinomen, Pteran, Nizbel...!
Ayla, {char1}, all!
Smells like all...!
Has smells of all earth's life!!

Ayla: This smell like Ayla's
land!
Reptites, Ptero, Nizbel.
Ayla, and [member1]!
Smell like all people I know!!...
Smell of all living things!!

Magus: ......
It's slept underground and kept making
the evolution of all life on this planet
its own since bygone eras...?

Magus: ......
So...since the dawn of time, it has
slept underground, controlling
evolution on this world for his own
purpose...

No mention of controlling evolution, just taking advantage of the results.

Crono grabs his sword hilt.

Marle reaches for her crossbow.

Marle: Then that means we've
been living for its benefit!?

Marle: Are you saying IT'S the
reason we're all here?

Lucca pulls out her hammer and holds it up in front of her.

Lucca: Taking advantage of the evolution
this planet's life took so long to go through
just by sleeping, that's way too selfish...!

Lucca: Grown like farm animals,
waiting to be slaughtered...
All of our history...
our art and science...
All to meet the needs of that...
beast...

Robo assumes a battle stance.

Robo: Combat ability value is....
C, cannot measure!
My sensors feel ABOUT to break!!

Robo: Attack value is...
...off the scale!
It's shorting out my sensors!

Frog brandishes his sword.

Frog: You've GOT to be kidding....
We're NOT all living so we can
feed you, damn it!

Frog: It...is too much to bear... We
have been reared like animals...!
Our lives hath been for naught...

Ayla takes an aggressive stance.

Ayla: Not run away!
Ayla not run away!
Run away worse than lose!!

Ayla: Me no run!
Ayla no run away!
Worse than losing!!

Magus: That means we were, so to
speak, no more than Lavos's feed.
We humans, no...
All life on this planet....

Magus: We were created only to be
harvested. All people...
...and all living things...

Crono whips out his sword and his hair blows dramtically in the wind (what wind?). As others have noted, he makes as strong a statement as any of the others, without even speaking.

Marle: This is Crono's and our...
Leene's and Don's... everyone's...!

Marle: This is Crono's and ours...
Leene's and Doan's...everyone's...!

Marle enters her casting stance.

 Everyone's planet!!

 ...and I mean, EVERYONE's world!!

Lucca: No....

Lucca: Impossible...

Lucca draws her gun.

 I won't stand for such a
ridiculous evolution!

 You CAN'T expect to toy with an entire
world and get away with it!

Robo: I am a life created by humans....
HOWEVER, life IS life!
Just like Lucca, and EVERYONE else....

Robo: Human hands created me...
Which means I am a product of that
thing...I am no different than
Lucca and the others...

"A product of that thing"? How so? Anyway, he's taking a stand as part of the planet's life against Lavos, so even if that were true, it doesn't fit here.

Robo thrusts his fist out.

 I AM one of the many lives on this planet!!

 I am a part of all living things!!

Frog: For the sake you scum...!

Frog: My life retain'eth its
meaning...!

Frog flips into a dramatic battle stance.

 That's NOT why we're living!!

 We haveth our own will!!

Ayla: Ayla not lose!
Aylas this earth's life!

Ayla: Ayla strong!
Ayla and world good!

Ayla takes a dramatic kicking pose.

 You not this earth's life!!

 You outsider.
Not part of planet's life!!

Magus: ......
I WILL beat you this time, vermin....

Magus: ......
It's over for you...

Magus does his dramatic pose.

 I will settle my long struggle...!

 Your life ends here!

Lavos makes the Evil Lavos Noise and holds up two somethings that float to the sides. The screen whites out and fades back in to show everyone in a warped dimension different from those previously seen. 3-14 ラストバトル (Last Battle) plays as the party engages the Lavos Core.

The bit on the left absorbs all elemental attacks and uses the incredibly annoying Seal Protection ability, so kill it as soon as possible with phyiscal attacks. The humanoid part has no unusual defenses, but uses the largest variety of moves, including a powerful slap and all of the following:

Changes the background, showing a distorted view of one of various eras.

Even though this doesn't do anything by itself, characters can still counterattack it. Feel free to abuse this.

A huge bolt of lightning strikes one character, inflicting non-elemental magic damage.

Physical damage to the entire party. Used only with the Primeval backdrop.

Slow status and some damage to all characters. Used only with the Ancient Era backdrop.

Hurts all characters, but at least it can't kill. Used only with the Middle Ages backdrop.

Hits all characters, and also inflicts minimal damage. Used only with the Present backdrop.

Magic damage to the entire party. Used only with the Future backdrop.

The bit on the right is nearly invulnerable at first, but that's simple to negate. When either of the other parts dies, it's not long before the following occurs:

If it's attacked at this point, it takes normal damage, and the following message appears:

After some time passes....

Any slain parts come back to life at full health. For a time after this, attacking the right bit gives the following message:

After some more time passes....

This makes the right bit nearly invulnerable again. The right bit is the only part that really matters. It has a lot of HP, but as soon at it dies, the battle is won. It screams the Evil Lavos Noise one last time, the background flashes through all the eras, and the screen goes white....