If this is before you get your Gate Key back: You gotta get through the caves by dropping down a bunch of holes; at that point in the game there's only one path to progress. There are ways around MOST of the bao baos (those big gorilla guys), but you're gonna have to fight a couple. They are the strongest things down there and also the only thing not weak to lightning magic. You may want to grind on Gato to get some more levels; especially if you have avoided most encounters.
Focus on the bao bao first. They take slightly more damage from magic than physical attacks. I like to have Robo here because he can dish out damage and do full party heals. Ayla and Crono are required here, but they also have an excellent duo tech that deals lighting and physical damage to everything in a wide area.
The boss at the end is a Triceratops and you'll get your Gate Key back while being introduced to another boss you'll fight a bit later.