rounded off screw head

[Robbied196]

You can put a small grinding bit in a dremel and cut a slot for a screwdriver.

 

[gregs656]

With hammering a Torx bit in, I'd be concerned about damaging the threads.

I wouldn't be, you only need to get it to grip, and a few shocks through it might loosen it up some anyway if it's got a bit of corrosion or what ever going on.

Have you been having goes at it with penetrating fluid at all? Might be worth doing that for a few days before trying anything.