Couple of options:
Get a star/torques bit and gently tap in to the hole and hopefully it wont be too tight to undo
I've had this problem with my current (second) and first Puma.
On the first one, I cut two flat edges off each bolt head and undid them with an adjustable spanner
The second one I cut a slot in the top and undid them with a flat screwdriver.
On both occasions, I used a Dremmel multi tool with a grinding wheel in it.
I think there have been other options, so I'll leave them to someone else!