All of the above- - - - -Kroil, impact, and heat. Kroil seems to work best if I apply it a few days before I really need to make the bolt move. Then use a back-and-forth action, stopping when things start to tighten up. Drilling/tapping a broken bolt as a last resort is a one-shot deal- - - -get the first hole drilled off-center and you're in deep doody! Set the part up solidly on a drill press or a milling machine if at all possible.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!