A sharp centre-punch and a hammer will almost certainly get that screw out.
In fact those screws are totally unnecessary and the wheel studs prevent the drum from coming off when the wheel is fitted.
I read a post of yours on earlier today where you mentioned that these screws are essentially useless and to just put some copper grease in between the drums and the alloys to prevent them binding so that's exactly what I'm going to do when I put everything back together. I'm going to try a punch tomorrow evening after work so hopefully that'll work
The screw ARE there for a reason. If you lose a wheel due to the nuts coming off or studs breaking at least you will still have brakes to stop the car! If the screws are not there then you risk losing the drum as well and total brake failure on the single line system or at least 50% if split system. Is it worth not fitting them?