In my opinion, even the so called good ending sucked. But I'll explain how to get it to the best of my ability for you.

First, be sure that Serge has the Crono Cross allocated on his element grid. It doesn't matter what level you put it on... in fact it would be better just to place it on level one.

Next, make sure all the characters who will fight the Time Devourer have plenty of level one elements of each color allocated.

When you go into battle build your characters stamina all the way up to 8. Then denfend until the Time Devourer attacks again.

Have Serge start off by using Uplift. Then the next character should use Fireball. The next one should use Bushwhacker, to be followed by Aquabeam, Gravity Blow, and Photon Beam. By now it should be Serge's turn again. Just use the Chrono Cross and the game will be over.

You must use elements colored in this order for the Chrono Cross to work: yellow, red, green, blue, black and white. If you mess up, need to heal, the Time Devourer messes up the order, or whatever else goes wrong, go ahead and do it, then start the process over again. If you have any other questions, I'll be happy to help.