I tank this every week and it's a complete joke.
The boss is facing you as you enter and you need to tank him where he stand, but turn him to your right so he is sideways to the rest of the raid. Put your camera at max range and look out of the lava waves popping up on each side, then aim for the gap inbetween the lava wall. (If you are doing it correctly, the boss should only rotate, but never actually move).
Tanking this is actually easier than any other role in the instance