Zalmranth the Colossus Bug?

I'm in Cruel mode now and the boss becomes invulnerable, no matter which angle I hit it from or part of its body I aim for, it doesn't get damaged. My minions are "hitting" it but aren't dealing damage to it. It happened to me multiple times and each time it happened around half health. I wasn't able to record it because my frustration prevented me from thinking about recording it.

Also sometimes I would dodge roll away from an attack or get knocked back and fall off the map, it's not that big of a deal for me as I can get back up instantly, but it's still a bug that I think needs to be looked into.