I think the easiest and fasted Way is in Chapter 23 "The Crossing".
Load this Chapter and go forward to the Billboard / Bus Point (its not missable). Go upstairs and then right to the Bridgepillar. Take the Helsing and follow the Compass.
You enter a Building and go upstairs to the Bridge. In the Top Room before you leave the Building you can watch outside and you see a Demon on a Fench.
When you go outside he fly instantly away, stay inside the Building and you have ca 10sec for a One Shoot Kill with the Helsing before he fly away. Easy going :)
PS: when you want, can you find more Guide here