Dragon Quest VIII/WMG

By giving himself a body of flesh and blood, Rhapthorne was Hoist By His Own Petard.
If the logic of Wild Arms 2 and Final Fantasy IV also applies to the world of Dragon Quest VIII, then by obtaining the body of flesh and blood he so craved, Rhapthorne made himself permanently destroyable. In Wild Arms 2, the Kuiper Belt was a sort of ephemeral force that consumed worlds wherever it went which not even the powerful Dragons could fight because there wasn't a form to attack. A plan was devised to defeat it by giving it a body of flesh and blood. By having a physical form to strike down, the Kuiper Belt was destroyed forever. The same is true of Final Fantasy IV where had to be forced into a corporeal form in order to be defeated. Rhapthorne was untouchable while he was still a soul in the staff. By giving himself a physical form, he made it possible for him to be destroyed! Oops!
 * Fitting Enough, villains tend to be arrogant. Probably Rhapthorne thought that noone could possibly beat him in his true form.

The Argon Hearts aren't the part of Argon Lizards you think they are.
First and foremost, the lizards you beat for them get up and run away when beaten, clearly indicating they still possess all their vital organs, such as their heart. Furthermore, it's far too crystalline and rock-solid to function as a blood-pumper.
 * Having played Breath of Fire IV and learned how Faerie Jewels, prismatic gems that regularly get crafted into swords, were made, this troper has a rather disturbing theory that what you're actually collecting is, in fact, the lizard's droppings. Talk about beating the crap out of them.