If you have any multi-hitting magic attacks (ice 2, fire 2, lightning 2) (ice sword 2/fire sword 2 won't do because niether of them reach far enough to attack multiple body parts) then use them as often as possible, but most importantly, keep your party's health and mana as high as they can get. If you have elixirs or megalixirs with you, that's a bonus (steal megalixirs from the fossil ape in Giant's Claw -- 600 A.D. -- west of Magic Cave, small island with a single mountain) Attack with those types of techs unitl both arms falter, and then use your strongest techs/attacks on the head, but KEEP YOUR HEALTH HIGH... mana can be kept high if necessary.

Ok What you need to do is make sure you have the three main elemental party members in your party, i.e Crono, Marle and Lucca. Attain their level two magic moves which are Ice 2, Fire 2 and Lightning 2. When you have gotten all of these you should get a triple tech move called 'Delta Force'. With this you can attack all of the body parts while keeping your health close to perfect because after you destroy his arms for the first time you can destroy them again and again without getting hurt until you either kill him or run out of mp. I did this first go but make sure your guys are at least on level 20-22 depending on character. I only got attacked twice and kicked his ass because df does around 1500 damage and Giga boy only has 10000 health.