This is necessary for any of the main variations without the Epoch. You'll destroy his first form and skip directly past the outer shell. When the Epoch is able to fly, take it to 1999 A.D.A time gate will appear to take you back to the end of time after you beat the outer shell. This isn't strictly necessary for any of the endings, and you will have to battle through all of Lavos's "pre-forms" before you fight his first body. and tell your party you're ready to fight Lavos. At the end of time, go into the bucket to travel to the day of Lavos in 1999 A.D.It's also the easiest way to obtain most of the endings since you will skip directly to Lavos's first form.
This is necessary if you want the Programmer's Ending at the start of the game or the Frog Ending.
If you challenge Lavos after Crono's death and the Black Omen's rising, the ending will depend upon three factors: whether or not you saved Crono, whether or not you spared Magus, and whether or not you lost the Epoch by ramming it into Lavos, thus allowing for eight variations total. The ending you receive after you beat Lavos depends upon what point in the game you are at, or what the save file reads when you save your game.
The Queen is Gone ( A Vanished Princess 600 A.D.