Hits for about 600.000 so can't be tanked. Must play aggro ping-pong with the wands that drop from previous bosses. A good place for the group is in the water (where the 2 streams cross) which sometimes prevents getting kicked, and if you land in the water again you wont get falling damage.
One tank stands at the edge of the cliff where the waterfall is, the other one stands upstream along the river, the group is right in the middle. The tanks "play" the boss like a ping-pong ball between them. When he reaches about 30m distance to a tank, give a signal to the other tank so he can pull him back again.
The group should be in the path, but not exactly let him walk all over them.
Takes about 10 minutes to take him down.
Every death of a player makes the boss stronger, so try not to die :)
Enabling "Show Distance to Target" in Options/User Interface/Name and Chat Bubbles is quite useful here.