apply some heat. Be careful though to not burn anything.
If you strip them, you will need to drill them out.
another is leverage. use a socket type tool that is "L" shaped, and apply pressure on the screw, while pulling on the furthest end of the handle. add a shaft to the handle if you need more leverage. the longer the handle, the more torque you can apply.
Good luck !
Last edited by American_Freedom; 03-01-2012 at 12:19 PM.