This guide (part 1) will help you defeat the bosses that you'll encounter on your jounrey in Chrono Trigger.
.01 Yakra (HP: 920) - This is the first boss of the game so it's pretty easy. The best way of defeating him is to use Crono and Frog's "X-Strike" and Luca's "Flame Toss".
.02 Dragon Tank (HP: N/A) - The "Dragon Tank" made of 3 parts (head, body and wheels). The best way to kill it is to start with the head as it can (and will) heal the other body parts. Once you have decapitated him, move onto to his wheels to prevent him from moving, then finish him of by killing his body.
.03 Guardian (HP: 1200) - The best way of defeating this boss is to start by killing the two "things" as he uses them to attack you. Once they are dead start hitting the Guardian with everything you have except for fire-techs as they don't have any effect on him.
.04 Krawlie (HP: 500) - This boss is pretty easy and can be defeated by only using physical attacks.
.05 R Series (HP: 150) - The "R Series" won't be hard to defeat. Use Crono's Cyclone a few times and you're done.
.06 Heckran (HP: 2100) - This boss can only be damaged by magic. Use Lightning and Fire techs and let Marle heal. Do not attack when it sits in its defensive position.
.07 Zombor (HP: 960) - Eliminate the top half first using Fire and Ice techs.
.08 Masa and Mune (HP: 1000x2) - Attack the left broter, Mune, once he's defeated the fight is over.
.09 Masamune (HP: 3600) - Use all your powerfull techs on this one. Use slash when it says "Storing Tornado Energy".
.10 Nizbel (HP: 4200) - Cast lightning then use all your best techs, repeat this a few times. Remember to heal.
.11 Flea (HP: 4120) - Let Crono and Frog use X-Strike while Marle heals.
.12 Slash (HP: 5200) - Use regular attacks until he picks up his sword. Remember to heal with Marle.
.13 Ozzie (HP: N/A) - Don't hit Ozzie, aim left and hit the switch. Repeat until he falls.