Defective/corroded bolt on Arnott strut

[fmugur]

I had a leaky air spring a while back and decided to replace the fronts as a pair.
The driver side had the BMW original strut which was leaking. The passenger side was an Arnott and I am not sure it was leaking too but I went ahead with the replacement anyway. Now, on this passenger side, the strut went in easy. I torqued the three bolts on top (engine compartment area) and then moved back underneath to finish up and put the wheel back. While working on this I heard a piece of metal shot up. It hit the hood and then back in the engine compartment. I looked around to figure out what just had happened and I didn't notice anything at that point. I thought maybe something from the street flew over but it was a quiet afternoon and nobody around me. Anyway, after I finished with the wheel and I went back and look for the piece of metal or clues. It didn't take long since I only touched three bolts on that area. I checked that first.
One nut and its own bolt were gone. I found it on the tranny heat exchanger valve. Here is where it snapped and why:







The good thing is that I don't have to take the strut out. The bolt is removable from the strut assembly and I can use one from the old strut.
The difficult part is that the bolt seems to be very hard metal and I cannot use normal drill bits to drill through.
Any suggestion on what bit I should use? Cobalt? I have a bolt extractor but I need to drill a hole first.