Look for an item called a Chrono Cross and equip on Serge and see what happens when you use magic. Also, there's a hallway in the final dungeon that has a repeated sound motif in it (music stops). Good luck getting the ending with these hints.

EDIT: oh one more tip. One the first playthrough, don't worry about the pacing; do whatever you want. At the end you get a newgame+ and also a continue+, so keep some saves handy for that so you don't have to replay everything if you want to get all characters. Although, it's good to do a newgame+ at least once to see the story unfold pretty quickly. Oh, and here's another tip. If you don't have Leah now, you missed her near the beginning of the game. Reject Kid at the beginning until she leaves; don't worry, she comes back.