If you can grab the head with a pair of pliers, that'd be the simplest way. Vice grips would probably be your best bet. If pliers won't work, then you'll probably have to get one of those special screw removal tools.
Two other options:
1) enlarge the slot with a dremel with a cutting disc, the use screwdriver.
2) Use a drill bit the approx diameter of the screw shaft and drill straight into the head. When the head pops off you can remove the dw front and then remove the rest of the screw by grabbing the shaft with vice grips.