[spldart]

stubborn screws.
Place whatever multitip bit that fits the screw head into the screw, with an extension if necessary. Then give a thwack to that bit/extension with something smaller than a hammer but bigger than a screwdriver handle. Sometimes I find the side of a 3/8 drive socket wrench or it's handle is good. The object is to send a jolt down into the screw head without deforming the screw head.
PS: If you have a partially stripped philips head screw then sometimes that 'thwack' on it's head can be delivered with a 3/8 extension. The blow will not only loosen the screws ninja grip but also deform the metal into the reamed out x ridges. Adding a little possible bite to your screwdriver.
Your results may vary.