These bolts can get really stuck. Mine was on so tight the
torx head shattered (it was a short one on a 1/4 inch socket
and extension). I was a bit mad but maybe it was better that
the torx head broke then having the bolt strip (as in your case).
I think you should try some penetrating oil and let that sit a bit
before the next attempt.

Also, be very careful taking the back bolt out and putting it back.
I dropped mine down into the engine (talk about everything going bad).
I ended up tapeing a small cylindrical magnet to about 4 feet of phone
cable and dipped it down until it found and picked up the bolt (that
was lucky, something had to go right after everything else went wrong).