I've had an immense amount of trouble with it as well, I ended up having to use a macro to get it to activate at first. While that worked fine and good and all, there's another solution.
The wiki contains information for you to rebind both the Devil Trigger and the Gun Special to a single key via editing the .ini files. I'd suggest you read through the page and follow the instructions there. It was very helpful to be able to only hit one key rather than mess with the timing to perfectly hit Z + C simultaneously.