Eirin is Selenium, because Selenium comes from Selene, the greek goddess representing the moon.
Flandre is Fluorine. Not only are the names similar, but Fluorine is the most reactive element, able to react with (and thereby "destroy") pretty much everything. Also, its natural state is the F2 molecule, which is relatively easily split into two radicals by sunlight, i.e. it is "weak" to sunlight.