I've seen a few comments about the phoenix necklace recently and wondered if it actually entirely prevented K0s. For example, if I have 50hp and someone ags specs me, is it possible for them to kill me with one hit or will the first cause my hp to go to 30% of max and then after that he can kill me?

Its like RoL if the damage is done before it hits the required amount you die regardless.

 

In your example you'd get KO'd, if it did less then 50 damage then it would activate.