Really quite simple and even readily available. Tracer ammunition. It's the stuff that you see flashing through tha air in all of the military movies. It's a regular bullet with a white phosphorus and thermite core, burning at a temperature in excess of 2000 degrees farenheit. A gouple of those out of a .45 automatic should incinerate any vamp or demon and would even give the average hell god fits.
They're available in a variety of calibers, but .45 acp for handguns and either .308 nato or .223 should handle anything a Slayer might run into on a routine night's patrol. For something bigger, there's .50 and 20mm machine gun rounds, and for the REALLY big stuff, whatever artillery you might prefer.
Let the mayhem begin.