With zhonya's, 500 ap (not too hard to max at) he's rolling like 180 armor at minimum, which assuming they have LW+cleaver+pen masteries/runes...
-30% from cleaver -> 180 - 54 = 126
penetrate 48% of that (ima just round to 50 to make math easier)
so 63 armor left, remove 34 flat from runes/masteries.
so 29 armor (31.2 or so assuming I would've done the 48% calc instead of 50%) which is still a reduction in damage.
Now assume he had no passive.
105-30% = 74.5
take off 48% and you're left with about 39 armor
remove 34 armor from flat and that becomes 5 armor.
So effectively it's the difference between 5 armor and 29 armor. I CBA to find the value decreases on those but it's probably something like a 4% damage reduction vs. 23% or so.
So 96% damage vs. 77% damage. 77/96 = 80%
So at 500ap xerath's passive will reduce damage by ~20% assuming the reduction percentages are somewhat accurate at the end there.
Do note that if they can't get all 4 cleaver stacks this drops a good bit, as well if they don't have a full pen rune page or if you have flat armor seals.