This is story related and cannot be missed.
After investigating the gunshots at the stage (killing 3 bikers in the process), the next mission will task you with saving further survivors being attacked by bikers (yellow diamond markers on the map).
I found the survivors were actually quite good at killing the bikers themselves, but just help them out, and you will then be called back to the hospital to kill the last lot. After this, speak with the lady doctor and achievement is yours.
Good luck!