First:
Take the battery out.
Second get a pair of vice grips (provided you have a replacement head)
Adjust the vice grips so the teeth dig into the metal at the top of the head, you can feel it when you do this. It will give slightly.
Turn the vice grips until the head unscrews.
If the vice grips slip tighten a half turn and try again.
Repeat as necessary until you get it out.