You mean the ending you get when you hop on the right telepod as opposed to the left one that Lucca tells you to stand on during the Millennial Fair?

Jump in with Crono and Marle. You'll definitely need to have completed the game once already as there's no way you can beat Lavos on a New Game file. I don't think it should be any trouble if you've finished the game at least once already but basically get Crono to Confuse/Luminare the crap out of everything and Marle to heal.