As posted. just drill the head off, then remove remaining bit with Vice grips. If that fails drill the remaining stub out with a 4.0 or 4.2mm (Max) drill and run a M5 tap down to clear the ****e out of the hole. Be sure to get the drill central on the stub.
You could try soaking the remains of the screw with Plus Gas or similar...Forget WD40 etc., it's rubbish at freeing corroded/rusted bolts & nuts.