Can't Damage Avatar of Silence in Zana map

I got the Death and Taxes map from a Zana daily mission and killed the ice and fire forms of the boss just fine, but my Cleave wouldn't hit the lightning form at all, even with Resolute Technique. I could apply blind from the Bringer of Rain, enfeeble from CWDT, and burning from herald of ash, but his health bar did not drop at all. He had immortal call up non-stop the entire time but even elemental damage didn't work. I was still hitting and killing the animated weapons normally. I tried /oos multiple times and I never seemed desynched.

Here's a screenshot showing his full health bar after several minutes of trying to hit him.

http://oi61.tinypic.com/n3qdcj.jpg

Is this a bug or am I missing some mechanic in the fight?

-Thanks

Pretty sure you're not finish taking him to where he's suppose to go..

Why doesn't Death and Taxes give you a less subtle hint. I remember when it first came out, people whacked it for a few minutes before they had the sudden inspiration to drag it to the portals.

Ah, I didn't realize you had to do that. It seemed like the first two forms died right where they spawned. Anyway, thanks for the replies.