[Zeke]

Don't break the screw off what ever you do. Heat is your friend, but only highly concentrated heat from a small tip of an oxy/gas setup. Use the impact going both ways, then try the leverage of the crescent wrench on a square shaft screwdriver. A big pair of vicegrips on a round shaft SD will do in a pinch. If you don't have the impact SD, pound and twist away, both ways until it cracks loose.

BTW, you're not always home free once it breaks loose. I've broken just as many bolts and cap screws on the way out as I have right off tight. Take it easy and use penetrant lube. Be SURE to clean off every trace of any oils with brake cleaner before reassembly. Just a smidgeon of antisieze on the screws will help in the future, but not enough to migrate onto the braking surface.