I’m not a stun expert, but I don’t think you can perma stun bosses. The most recent 1.1 patch included provisions to prevent this, such as the following in the patch notes:
“Other Boss Changes
Bosses have 50% more effective health against stun and freeze
Added modifiers for effective boss health against stun and freeze and changed freeze chance to take ward into account.”
You can still stun them, but it’s become more infrequent.
