Last few playthroughs I've actaully been Running Crono, Magus, and Robo.

Just split up your Magic tabs between Crono an Robo, and now you have 3 people who can hit all enemies for high damage, (Shock, Luminair, and Dark Matter) And you can get Robo's Heal Beam to cure a lot of life.

Just give Magus your Safe Helm so you can cover his small defense. Robo and Crono have a good enough defense to without it that Robo's Heal Beam is more then enough to keep you running.