Derrick is the Prologue level Boss, so when I beat him there and this didn't unlock I thought it had glitched!
However, during the final mission 'The Queens Eye's', you can find Derrick again and by beating him here will unlock you this Trophy.
At the 2nd Shrine, head down stairs and he's in a torture-type room - his main attacks are a rush-charge, a great-axe that comes from different angles with ridiculous range, and a defensive move where after a few hits he emits a damaging red explosion.
Be careful here, as you can get stuck against various non-breakable objects.
I beat him by dodging his charge and getting 2 or 3 quick attacks in, then retreating. The odd arrow and rifle shot worked nice as well.