I encountered my first Head of the Beast this morning in The Hobgoblin Horde. The mist pocket looked small so I did not back up. When it appeared, my first thoughts were, "Oh. I have not seen this one before. I wonder what it does." When Albus Gladwin received a cursed wound and I looked more closely, I realized it was the head of a hydra.

Then I ran. I was in the part of the dungeon with the ogre and ran into a dead end.

The monster chased me. Then, because of the mist, I ran into another dead end.

The monster chased me. I brought up the map and still managed to get into a dead end.

Keep in mind, this area of the dungeon is really small with low ceilings. It was funny. My hire and I both had cursed wounds. His hit points were almost gone. Mine were full. I finally stopped, threw up a dimension door, took a couple hits, and jumped to the entrance.

My hire also made it out alive.
To my surprise, it was far from my most harrowing encounter with a Mist Stalker. To my credit, taking Dimension Door really was a good idea.
It looks like my best bet is running The Borderlands over and over for lantern points and, if I get super lucky, a piece of usable equipment. I also went back to the Demon Sands since the chests are off ransack. Those quests are really long, though, especially with a character that cannot do much damage. The Chamber of Raiyum took an hour to clear.