If all else fails and they have to be drilled. Drill the head out not the actual bolt ie only go in to the tapered depth by upping the drill size diameter, sonner or later it will break away. Keep the revs down it will generate too much heat.
Horrible things those hole type, had them on my last car. Noticed a few times whereby i have tightened these bolts up, onl to find a lot more leverage is necessary to undo, its the chemical reaction of steel to alloy. Copper grease may help in future.