The
wiki page greatly helps when trying to find him, and is what I personally used.
It's important to know that (as with all ending achievements) you can get this is an existing save, which greatly helps since he only appears after all three Dreamers have been destroyed.
You'll also need this special piece of gear:
to hear his dialogue and thus complete the achievement. (without it equipped, the text just shows as gibberish)
PS: Ending-based achievements (such as this one) won't trigger until
after the cutscene finishes.