Okay, this is the easy way to beat Magus, he can actually be quite simple, if you follow these steps, be on at least level 22, that was the level I was at, and have Frog, Marle and Chrono, in your party, bring plenty of ethers, and follow these steps. You need to have, Heal with Frog, Chrono and Marle should know the Duel Tech Ice Sword and Marle should have haste.

1. Cure Everyround with frog, no matter what, use his heal ability and when you get low on hitpoints use aurawhirl with Chrono and Marle it'll save you life when Magus does his DarkMatter spell. Whatever you do don't attack with Frog, keep healing and if he runs out of mp, make him use a ether and have Marle and Chrono Heal instead.

2. At the beginning, make it so Magus barrier makes it so you can use water or ice and use ice sword, it takes about 600 damage away form him, maing his barrier change until you can use that duel tech. Now keep it up until his barrier breaks.

3. Once his barrier breaks, attack with ice sword and heal with Frog, when Magus uses his Dark Matter spell, use Aura Twirl and heal, keep on doing this and you'll whip his butt big time, and my charcters didn't die at all during this fight.