First of all I usually fight Culex around lvl 30 but now I found an easier way.

If you don't know how to get to Culex here are the steps 

1 Have access to Monstro Town
2 Go to Moleville and buy a firework from one of the north building.
3 Talk to the girl standing near the building and trade the firework for a shiny stone
4 Go to Monstro Town and hit A on the locked door 
5 This should take you to Culex but he'll still be tough for you

So here is how I do it 
1 Make sure Peach is in the party
2 Make sure she knows Group Hug
3 Be sure to block Cuelix's Normal attack 

Now you'll need the Lazy Shells here are the steps to get them
1 Beat the plant in Bean Valley and before going up read the note and get the seed do not sell it
2 Beat Nimbus Land story and go to the right until you fall of screen and see the shy guy. He'll give you the fertilizer 
3 Go to Rose Town and then when your there go north and east until you see an opening.
4 Enter the opening and you'll see a house
5 Go in the house and talk to the toad,He'll plant the seed with the fertilizer growing a bean-stock
6 Climb the bean-stock and you'll see two blocks, hit them and get the lazy shells. One is for Mario the other is for Defense for everyone
Note: The Defense Lazy Shell lowers your attack and Mg attack

After this you'll equip the lazy shell defense to Peach as you want her alive since she has healing
.Note: Make sure Peach has bathtub ring you can buy those at Marrymore And I believe it makes damage weaker
 Now fight Culex all the attacks by the crystals will do 0 damage or 1 or 2. But Culex's Normal attack will do about 60 to70.
Keep healing with Peach and keep restoring FP or use mushrooms to heal if you don't want to waste FP.
Most of the Crystals will lose their FP the last one being the Fire Crystal.
Note: If you have Bombs and Rock Candy use them as they do more damage.
When Most of the Crystals are dead or out of FP revive everyone with Pick Me Ups or Come Back and then you'll definitely win

Although many people who play this game consider using the Lazy Shell as cheating. I kind of agree. Yes, it isn't a code or anything, so it is fair game, but the Lazy Shell armor is INSANELY overpowered in defense. It makes it pretty much impossible for anyone who is able to get far enough into the game to get the Lazy Shell to lose. You don't have to be particularly competent in the game to beat just about everyone when you have someone with this armor. I think that it was not intended by the designers for the item to make everything as easy as it did. I personally don't find it fun using the armor because there is virtually no challenge to the game with it.

It is free experience though

rcarter2 : Download the Lazy Shell Editor program and a copy if the ISO and edit Mario RPG so that you create a more difficult and challenging version of the game. It's kinda fun. Vizzed would be a little more cool if things like that program were included here within the community too xD

Fireproof : Would be nice if someone could just make a hack of the game that just lowers the defense boost of the Lazy Shell armor a bit. I actually have no problem with the Lazy Shell weapon. It is only slightly stronger than the Ultra Hammer, which is the strongest Mario Weapon aside from this hidden weapon.