You don't need to shatter, just lure assasins. It's same thing since assassinate only deals piercing damage. Armor is just your total pierce crush and slash resistance.
Example:
Resist slash: 200
Resist crush: 200
Resist pierce: 200
Your total armor is 200.
So, you don't need to shatter, and since lures don't deal damage, this can be achieved on any mob.
i may be wrong but i thought that armor IS your piercing/slashing/crushing resist
for example:
if you have 200 armor, youll have 200 slash/crush/pierce resist
but then when someone shatters for lets say -100 armor, the resist will turn to 100 armor = 100 pierce/slash/crush resist
assassins lure is then added on top of shatter
so lets say the lure is for -100 pierce resist
then an additional 100 will be taken off of pierce alone (making it 0)and slash and crush will still be at 100
so two of them actually DO help, i think