I never used the Lazy Shell armor; it made whomever it was equipped to have no Attack power whatsoever, and I think it made their Speed majorly drop, as well. The infinite-defense thing just wasn't worth majorly sacrificing the other things.
What I think I did was equip Peach with the Royal Dress armor instead, and use the Safety Ring accessory, which blocks like all Status attacks, including damage with them, and I think gives decent overall defense, as well. I like that better than the Lazy Shell armor.
I used Mario, Geno, and Peach; make sure you have their strongest Weapons (Lazy Shell WEAPON for Mario-GET THIS. Don't forget the Frying Pan in Moleville for Peach, either) and strongest Armor available, and figure out what Accessories would be most practical.
Basically, don't use HP-recovery items; use Peach to heal allies when anyone gets low. Don't waste FP using Special Attacks other than Peach's healing moves and Geno Boosts ('nother reason I prefer using Geno over Bowser), unless it's a particular weakness for an enemy (like Fire Orb on the Water Crystal, or something).
MAKE SURE YOU HAVE LOTS OF SYRUPS TO RECOVER FP. This method is a lot cheaper than buying tons of KeroKero Colas.
If an ally falls, use Peach's Comeback on them. Be sure to have a few Pick Me Ups on hand in case Peach falls; only use those on her.
...So, anyway, that's my method, and I always did fine when at a decent Level. I like it a lot better than the Lazy-Shell-armor-on-Peach method, but that's me.
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fools, because they have to say something.