The enrage timer is fine, I think the boss should just hit a bit harder to make sure zerker is like, more of a high risk high gain sort of build really.
I also think that in this case either the boss has to much health or the timer is to short, this is to ensure players should be able to play with less risk, so to say.
Just how I feel.
Zerker should be a glass cannon in my opinion, just as nomad should be a concrete wall in terms of durability.
But overall the fight was enjoyable really. I love the druid.