Mercenary deals no damage to monsters with shield crush

I've noticed that my Bastion mercenary of level 69 usually deals 0 damage to monsters (but sometimes somehow oneshots them).

Video example of doing no damage against lvl 45 zone rare then getting a random hit from nowhere(maybe tempest shield): https://www.youtube.com/watch?v=-86CHTePliQ

Following video shows that only shield charge deals any damage
https://www.youtube.com/watch?v=Bk_628orjxQ