I would be a little careful with penetrating oil/heat as you may dissolve/soften the plastic used for the AC cover. The screws are a #2 Phillips size and the large screw driver that you are using just may not fit. Since they are going into sheet metal, that is not the strongest, an impact driver may do more damage than help.
I would make sure that you have a good quality #2 Phillips driver and apply a lot of pressure as you try to turn the screw counterclockwise to get it loose. Failing that, drilling off the head is all that is left. Once you have the old screws out and the holes cleaned out, replace them with some stainless steel sheet metal screws to minimize future problems.