First, find as many megalixirs as possible, steal from enemies constantly, if you have to, at least 30 megalixirs to be safe... elixirs or full tonics are also acceptable, so long as you have ethers. Use techs such as Fire 2 or Ice 2 or Lightning 2 (Ice sword2/Fire sword 2 aren't that great because they don't hit everything in one shot, not enough reach) Keep using those techs until both arms are destroyed, but KEEP EVERYONE'S HEALTH AS HIGH AS POSSIBLE AT ALL TIMES!! if you don't have those spells yet, then just use your strongest techs/attacks to kill the head. The arms will regenerate, so if you feel the need to keep destroying them, do so... they are a nuisance! Good luck, and remember... keep your health high.

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.