Sometimes I get a Dodge truck with alloy wheels in for brakes and their rotors corrode like sodium in a lake. You can user the holes that hold the caliper bracket to the knuckle to help. It works well on other vehicles as well, but with less force.
If your brackets are threaded and the knuckles aren't:
First remove the calipers and brackets (pad holders or whatever you like to call them.) Then, find a bolt that's just barely a small enough diameter that the threaded end fits through the hole in the knuckle loosely and just long enough that with the head of the bolt against the rotor you can fit into the hole. Get two of this bolt, two nuts to fit, and two washers as well. Get also a flat piece of metal longer than the distance between the two holes.
Onto each bolt first put the nut (all the way.) Then put the washer on. Slip each bolt through one hole with the head end against the rotor and the washer against the knuckle. Now use one wrench to hold the head of the bolt ant another wrench to turn the nut so that the head of the bolt is almost against the rotor. Set the flat piece of metal against the rotor (not needed if replacing rotor) to act as a shim. Turn your nuts so they force the bolt to press somewhat against the rotor using flat piece of metal as a shim. Now, at the side of the rotor furthest away from the bolts, give a couple of good shots with your nice hammer. The rotor should jump out just a bit and the bolts will not have as much pressure on them. Back off the nuts a bit and remove the bolts, then tap the rotor gently with the hammer and remove.
If the knuckle is threaded and the bracket is not:
Find the size (diameter and thread pitch) of the bolts that hold the caliper bracket to the knuckle. Get two bolts this size but much longer.
With the bracket removed, screw the bolts in until they touch the rotor plus three to five turns. Five works better but can damage the rotors.
Strike the rotor with a hammer a couple of times, then remove the bolts. Tap the rotor lightly with the hammer then pull off.