Okay
It's not exactly a monster.
And it's not exactly farmable.
But theoretically this thing can spawn indefinitely and gives almost 17M BASE experience, in the video I had rested exp so I got double and no exp boosters active, but you get the point.
If only someone would figure out a way to "indulge" the boss to make these things spawn rapidly, you could easily farm hundreds of million of exp relatively fast...I wonder if there's a way 
