Crushing blows is entirely based on the attacker's weapon skill and the defender's base defence.
In Tbc you could be crushed if the attacker had 15 more weapon skill than you had (base) defence skill, and that would mean that all mobs 3 lvls above were able to crush, as mobs got 5 weapon skill/lvl by default. However there were some mobs that were designed with highter than normal weapon skill, like the casters in Magtheridon's lair, (and I hear the same is true with one of the adds before thadius).
When the 3.0 patch came out they changed the amount of (relative) weapon skill one needed to land a crushing blow, from 15 to 20, effectively making only mobs 4 lvls highter than you able to crush.
i'm pretty sure blizzard will eventually make a boss with extra weapon skill, or have a mob 4 lvls highter than the players at some point. So in that case we would most likely need to get 103.2 to our "new uncrushable".
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On wowwiki I found an old formula that let you calculate the chance of a crushing blow.
Chance of crushing blow (tbc) = ((attacker's weapon skill - defenders base defence skill) * 0.02) - 0.15
After looking at that I figure that they just changed the 0.15 to 0.20 in the 3.0 patch. and if we use that formula we can find out how hight chance a 200 reduction in defence would be.
Chance of crushing blow (wotlk) = ((attacker's weapon skill - defenders base defence skill) * 0.02) - 0.15
((415 - 200) * 0.02) - 0.2 = 4.1
410% chance of a crushing blow. This means that every single attack that isn't missed/dodged/parried/blocked will be a crushing blow
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