at autozone an impact driver might cost you 20 bucks. I've had mine for years and used it on a lot of stuf. Just make sure you use the bit that fits best. There's usually a bit ass phillips and a more diamond shaped one. Just get the driver adjusted the direction you want it to go, put it on the screw, and beat that fucker with a hammer. You'll know when its loose, generally 3 or 4 good whacks and you'll be able to turn it with the handle of the impact.

 

impact driver best, couple o wacks-(make sure setting is correct) and it'l turn loose...we've had stripped ones at the shop...we would use a center punch to make a dimple spot then slowly tap in the counter clock direction. ive seen my mechanic set a hammer (ball side) next to the screw head and wack the hammer with another hammer to relieve the pressure