I have had a tough time removing hex-head flush mounted screws like the rear motor mounting fasteners on the 2012 Zero. If I can grab the head of the screw, I can use the tapping method, a pair of vice grips on the head or a number of other devices to remove the fastener. However, those flush mounted screws are tough if they get stuck. It is easy to strip out the hex head and then you may have to start drilling and use a bolt extractor to remove the damaged screw - and then find a replacement the fits. (I had no luck searching the Zero parts fiche at my dealer's parts department for a replacement. The screws didn't seem to be listed in the parts diagrams.)
My solution was to buy an expensive set of SK metric hex bits that fit perfectly into the hex slots, as any wiggling will strip the head. But before trying to turn the screw, I first spray some lube like Liquid Wrench or PB Blaster on the fastener, then whack it a few times with a hammer and punch to loosen it up. Then I finally carefully insert my bit, verifying that it is fully and securely placed into the hex depression and slowly turn the bit with a wrench making sure that it does not tilt while being turned. This procedure will work to loosen the screws every time that I have tried it.
Then what you do is to take the removed screw and go searching for a new one at a hardware store and coat the new fastener with anti-seize compound before installing it.