PTR Fatboss video is posted in the announcement thread above (along with all the other boss videos).
Ice Veins write up
It's kind of a whole boss fight out of the first phase of Blast Furnace.
-Blood lust at the start.
-Kill mini-boss Mar'tak ASAP, but keep this DPS priorityin mind first:
1) Siege Vehicles named Crushers or Artilleries
2) Felcaster/Gorebound Terror (switch and nuke as soon as it becomes a Terror)
3) Berserkers
4) Other siege vehicles
5) Engineers and Dragoons
-Interrupt the Engineer's "Repair" ability (just like in blast furnace)
-Tank swap on the berserkers b/c of debuff.
-Run ammo that drops from destroyed siege vehicles to the Cannons in the back of the room (like bombs to the regulators in blast furnace)
-Win.