Patty-
Are you talking about Culex? If so, try using Mario, the Princess, and Bowser in your party. Then, try equipping the following-
Mario-
Lazy Shell weapon (must have!)
Hero Shirt (or Super Jacket if you have it)
Rare Scarf
Princess Toadstool-
Frying Pan
Lazy Shell armor (must have!)
Safety Ring (must have!)
Bowser-
Drill Claw
Heal Shell armor
Ghost Medal or Jinx Belt
During the battle, just have the princess huddle into defence position, while Mario and Bowser duke it out with the crystals and Culex. When Mario or Bowser die, use Princess' Come Back spell to revive them. With the Lazy Shell and the Safety Ring, the Princess is virtually indestructible.
Now, as far as attacks go-
Use Mario with the Lazy shell to do the most damage. If you got the Star Egg from the Grate Guy's Casino, have Bowser use that every turn. It takes a long time, but he can effect at least 50 HP damage every time. Also, attack the Crystals/Culex in this order:
Wind Crystal (Green one, 800 HP)
Water Crystal (Blue one, 1800 HP)
Fire Crystal (Red one, 2500 HP)
Earth Crystal (Yellow one, 3200 HP)
Culex (4096 HP)
*If you have any Red Essences, use them all on Mario in this battle. Make sure one Essence wears off before using another.*