I believe the Lunasaurs only use the scary Bio/Reflect strategy if you use physical attacks on them. Try fighting them with just spamming Bahamut and see if that works any better (though I think they'll use Bad Breath after a point if you try this strategy). But if you're in the 50s, it should be bearable.

Also, as said previously, just run from the encounters on the crystal floors. Edge's Smoke Ninjutsu or Teleport can get you out of the battles quickly.