There are a couple of things you could try.
Try putting some grinding paste into the knackered allen hole and then using the original correct sized allen key to undo the bolt (this rarely works)
As stated, cut a flat edge through the bolt head and use a flat edge or impact driver to try and shift it a bit.
remove all the other bolts, drill the head off the knackered one (using small to larger drill bits in the centre of the bolt), remove the cover, use some mole grips to on the now showing bolt shaft.
Good luck