I had my alternator apart last week.. there are 3 philips heads that hold a bearing in on the inside.. nothing would budge them.. then I got out the long driver - pop - like butter!
as long as its on something relatively solid, I have an impact screwdriver set. you put a bit on it and hit it with a hammer. just gotta be careful what you use it on, cause if the screw doesn't turn it can usually rip the head off.