Don't give the Lazy Shell Armor to Mario! It cancels out the Lazy Shell weapon, so give the armor to Peach. Otherwise, that's a good strategy.
What I do is use the Star Egg since it does 100 damage on everyone, no matter how weak the person using it is (Peach), so you eventually knock off the crystals too while still doing damage to Culex. The Earth Crystal takes forever to go (I think it actually has more HP than Culex, but I forget) but like they said, it runs out of FP eventually, and actually so does Culex himself. I suppose once you get to that point, your stronger characters should attack normally if they can take more than 100 HP out of Culex on a turn, to speed up the battle a bit, because otherwise it takes a very long time to beat him.
"We took pity on him because he''d lost both parents at an early age. I think that, on reflection, we should have wondered a bit more about that."
--Lord Downey, Terry Pratchett''s "Hogfather"
