Hi.
Also, like to ad my two cents worth, I have had similar problems with stubborn screws.
The best to my knowledge, and the best tool for that job is my "Impact Driver". It will save your screw heads, and patience and accomplish the job of loosening the screws.
And well worth the investment, and can be purchased in any auto store or hardware store, or tool store.
It's a simple handheld tool the size of a screwdriver, and turns upon impact with a hammer blow.
It's great around the house, motorcycles, large and small motors, and equipment.
Tom