I usually use the item that makes you invincible for three turns (from most stuff, I think the dark star can still kill you). Can't remember what it is called. Always bring Geno, though, since his cannon attack can do some good damage on all the crystals. I also bring Princess and have her use the bomb attack/heal. Mario uses Ultra Jump or items. Once one crystal goes down it gets a bit easier.

Another strategy that I have read but not used is equip the Lazy Shell armor on Princess and have her heal while the other two attack. Apparently her def is astounding with it equipped. Cannot confrim this, however.