If you are playing DS version (which I assume you are) I had exactly the same problem. I'm a bit ashamed to say that I only solved the situation by leveling up like there was no tomorrow. The three weakest member in your party must have at the very least 3000 HP each except for Fusoya. Do not berserk Cecil as he can be useful as a healer. Have him use Cura at every turn so when Inferno comes he or Rosa will heal the injured as quick as possible. If you can, cast Shell on all of your party members (can easily be done at the start of the battle). Do not bother too much on reviving Fusoya if he dies, because he'll never go over 2600 HP anyway and he's most likely to die again and again,, keeping other party members from doing something more useful. And don't use magic to resurrect people; use Phoenix Downs, it's much faster.