Here's the facts + math as I understand it...
Each time you hit, the game calculates a chance to stun.
If the chance to stun is below 20%, then you will not stun.
In order to get a 20% chance to stun, then a melee skill needs to deal 8% of the mob's maximum HP with their hit. Dealing damage above 8% will increase chance to stun. This is with 0% enemy stun threshold.
A negative enemy stun threshold reduces the 8% threshold.
A positive enemy stun threshold increases the 8% threshold.
A mace version of the Boneshatter build has about 91% reduced enemy stun threshold.
This revised stun immunity archnemesis mod has 500% increased enemy stun threshold.
This means the mobs have 500 - 91 = 409% stun threshold.
This means you need to deal 8% * 4.09 = 32% of a mob's HP to have a 20% chance to stun it (without Ruthless support proc). Boneshatter runs Ruthless support, so every third hit does 98% more damage, which means you'd have to deal 8% * 4.09 * 0.5 = 16% of mob's HP with a ruthless hit to get a 20% chance to stun.
I suspect it'll be no problem to stun blue and possibly even rare versions of these mobs in normal mapping cirtcumstances, especially once you have good gear. However, good luck stunning a Harvest mob with that mod. Certain mobs just have very high maximum HP and dealing 32% of one of those mob's HP seems completely unrealistic for most of the character's progression.
Is my math wrong? Someone let me know.