The Quake Tortoise has the highest defense in the game, so to beat it you'll need a way to get around that. The easiest ways to do that are offensive items, all of which deal exact amounts of damage, or the poison and burn status, which deal a percentage of max HP. Besides those, there are other skills that ignore defense, like the "Blast" skills you can learn with tomes. And finally, though it's defense is very high, if you lower it's defense and raise our own attack enough you can deal damage that way, especially with magic.