If you're referring to the hand-held propane torch with the screw-on head, like you buy at the hardware store, go back to the hardware store and buy a similar-sized canister of MAPP gas. Most of them will be bright yellow. MAPP gas generates a lot more heat than propane and will get most areas of your guard almost white-hot in seconds.
A word of caution: if you're using good-quality wax castings, that's fine, but cheap wrought iron or pot metal, both of which get used occasionally for guards and buttplates (though none available from reputable suppliers, that I know of), just snap when you heat and try to bend them.
Take a few moments to visualize how you want the guard to bend, and where, before applying heat. Even the best-quality parts will eventually break if subjected to too many bendings. Try not to heat the area you're bending any more than you have to. Keep the torch moving, at least slightly, rather than holding it on one point, if at all possible. Don't put a lot of force into trying to bend the steel -- when it's heated sufficiently, it'll give pretty easily, and if you're already muscling down on it you're liable to bend it way, way past the point you want. Better to bend gently, a little at a time, until it's right, than go overboard. It's the back-and-forth flexing that will break the metal the quickest.
