Well, I imagine the meteors that fall during the ever shower are related to the magic that grants them long life and youth. As meteors tend to have high metal content, perhaps weapons forged from this meteoric metal can kill them.
Also, magical attacks. Since they all have some sort of inherent magic themselves, it makes sense that they'd be able to use their powers to kill each other, and also be vulnerable to other types of wizards, witches, and sorcerers. These two combined would basically mean they're resistant to most weapons and physical attacks, but can be killed by magic and weapons made from the special meteor metal. This seems a bit more balanced to me.
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