I had a cylinder head to remove and bolts were really tight. Three ended up with rounded corners so the only alternative was to drill down through the top of the bolt, open out the hole, weaken the head of the bolt and shear it off. So all I had to do was put a pilot hole down the centre of the bolt. I tried without a drill guide but I was way off centre. So I made a set of drill guides that fit inside the socket. That made drilling much easier. I had 12mm bolts and drilled down in stages ending up with a maximum drill size of 10mm. One bolt head popped off on its own due to the tension in the bolt the others sheared off easily. The heads had enough flats left for a hex socket to apply enough torque to shear.