Exactly. Why should a bebilith be able to smash a suit of full plate with a single attack roll, when even a colossal dragon can't?

If you want to make the bebilith scary, give it a Su ability to ignore armour and natural armour bonuses to AC with its attacks. Or give it energy drain. Both of these will ensure it gets lots of respect, without having to resort to a clunky system-breaking mechanic.

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