If your in the basement, and you have gotten through the undead and entered the 4-way with the golems staring at you. Take a right through the caged door and you will see a variety of mobs. He can spawn from about any of the skellies in this room. I've seen him right as you enter, secluded on the left, and on the far right of this room. There's about 5 mobs in here. 2 are not undead.
Clear the golems and pull to center if you need to, but be aware that golems can spawn an additional named.