Culex, to whom you refer, was easy if you knew what to do. Make sure you get the Safety Ring from the Sunken Ship and the Lazy Shell armor from Rose Town. Put both of these on Princess and go into battle.
Culex and his crystals will quickly wipe out your other two people, but will do only 1 or 2 damage against Princess per round. The only attacks that you need to look for would be Dark Star and the normal attack, both of which Culex does and are only around 30 damage. The crystals matter not.
Just keep pounding on Culex, you won't do too much since the Lazy Shell armor drops your attack by 30. Eventually the enemies will start running out of Flower Points. The Blue crystal usually goes out first, followed by Culex, Green, Yellow, then Red. After Yellow goes, (Red takes some time to exhaust), revive your buddies and just keep pounding away on Culex, he'll be dead in no time. Simple.
Btr'dSqrl