If used carefully, a diamond file, or wheel, can be used to cut or sharpen flints. But the danger is taking off too much flint.
The easiest way to sharpen the edge is to install the flint in the cock, so that it is between a 1/16" and 1/8" from the frizzen face, then raise the frizzen up from the bottom with the side or your left thumb, until the edge of the flint will strike the bottom of the face of the frizzen, or the " heel " if the frizzen if you look at the frizzen as a miniature leg and foot.Hold the frizzen at that raised spot, cock the hammer, and pull the trigger. The flint will strike quite hard, but at such a steep angle or refraction that the entire front edge of the flint will be struck off, all across the face of the frizzen, so that when the flint strikes the frizzen when you are shooting, the entire edge is cutting steel from the face of the frizzen, and not a few high spots. High spots create vertical gouges, and that eventually ruins the frizzen, or so reduces the number of sparks you get that you need to grind the face of the frizzen again.
If the angle that the flint strikes your frizzen is less than 55 degrees from the square on, you will get horizontal gouges, like the ribs on an old washboard. That will destroy expensive flints, and keep sparks from being thrown into the pan, and result in either hang fires, or very slow ignition, relatively speaking. You weill have to grind the face of the frizzen to restore the smooth surface.
So, I don't recommend using brass hammers( I made one before I got smarter) or other tools to knapp a flint. It is best done when held in the cock, so that the edge is married to the frizzen its going to be striking.