So I need to whittle him down to a sliver of health and then fill up his endurance bar? I just filled up the bar and he died instantly.
The Healthbar of the boss at the end of the fight should look something like this:
There needs to be a bit of health left but the blue bar needs to be filled, you can achive that by beating the boss as fast as you can.
After that another cutscene should play that starts like this:
For video help, here is a lets play where someone achived it (around 4:50 in the video):
hope this helps!