Chrono Trigger Developer's Ending

I don't know what level should be the minimum for attempting this, but anything where you have max HP/MP without the use of items and all required weapons/armor/accessories ought to do it.  You'll only have Crono and Marle for the fastest route to it, so you only have to worry about them.  Also, you need some MP-restoring items (HyperEthers make it easy).

The setup for Crono is as such:

Weapon: Rainbow
Helm: Haste Helm
Armor: Nova Armor
Accessory: PrismSpecs

The setup for Marle is as such:

Weapon: doesn't matter, she never attacks
Helm: Safe Helm
Armor: PrismDress
Accessory: up to you, I used the Speed Belt since I didn't have her speed maxed out

Start up New Game +.  Head to the Millenial Fair, bump into Marle, get pendant, Marle joins party.  Now set up equipment, and set the menu and battle cursor position remembering on.  Talk to the guys at the entrance to where Lucca and Taban are preparing their teleportation machine, then head south one screen and talk to the woman sitting on the fountain.  She'll talk about Lucca and Taban's show and then say they should be ready by now.  Go back up there, wait for Marle to choose what she wants, and then hit the sparkle on the right pod.

Lavos will appear right away, and Crono will be ready.  Cast Luminaire until the first form dies.  If you're lower level (or rather, Crono's magic stat is lower), this may take a while, but at 65 magic (where he was when I did it), two Luminaires will seal the deal.

Run through the passage to the second form of Lavos.  Continue your Luminaire spam.  Keep Marle ready, and have her refill Crono's MP as needed.  Eventually the second form will die, and it's on to the third and final form.  It's important to finish this fight with Crono having more than 12 MP.

Right away in the final form, have Crono use Confuse on the left bit.  If his attack power is high enough it'll die instantly, if not, repeat until it does.  Marle should continue to keep Crono's MP full.  Once the left bit is dead, wait for the Lavos core to shut its defense off and then resume Luminaire spam.  Eventually the left bit will come back, so kill it again.  Shortly after this you should win.

Then it's on to the ending.  You have to talk to everybody.  There's a battle in the prison, have Marle use Ice 2 and if her magic is high enough it'll end the fight immediately.  Once you've talked to everyone, return to Gaspar and he'll open the door to the Dream Team.  Once you've talked to all of them, the credits will roll.

The equipment setup does have one caveat: Marle doesn't have status protection.  At one point during the fight with Lavos' second form I had to have Crono use a Heal on her.  Whether or not status effects get inflicted is purely luck.  If you're having a hard time with it, swap her helm for a Vigil Hat.

Enjoy.