Guild Wars 2

So iv got to the shadow of the dragon end quest as a Guardian ,n the 3 phase fight.
I get to phase 3 each time, and no matter what i do to attack the Head, all i get is invulnerable?!!
Its literally taking no hp away and its killing me going through the cycle!

Any Suggestions?!

i have yet to find a way to get melee attacks to hit the head at the moment, you would likely be able to hit him with the 1 skill with staffs, but outside of that, i dont see a way to hit it without range