tl;dr - BONK.

NOTE: This requires nearly EVERY source of "increased stun duration on enemies" in the game in order to achieve a duration that leaves wiggle room for monsters with stun duration resistance.

https://pastebin.com/3qk0rCbh

This is naked, just to prove the point that the stun duration required is achievable. New Warlord's Mark garuntees us 100% chance to double stun duration, which is the only reason this now works.

The weapon equipped is a terror maul, which isn't necessary if you have both cluster jewels. There's some wiggle room in stun duration here where you can probably use Tidebreaker and Stun support, which might be required to stun Shaper, Elder, etc. due to stunning monsters with very large life totals works.

Prerequisites: 450+% Increased Stun Duration on Enemies, 100% Chance to Double Stun Duration, enough damage to actually stun something.

This assumes most monsters worth talking about have approximately 30% less duration of stuns. I don't know if that's the case, but it's a fair assumption that bosses have stun duration resistance.

With this, just your Ruthless stuns will stun long over to supersede the 4 second stun immunity of bosses. Non-ruthless attacks have enough stun duration to nearly permanently stun bosses.

This is not min-maxed in any reasonable way, it's got some weird stuff that was just filler (like having RT and the accuracy ascendancy).