Part of the problem is that there is already a tool broken off in it...Not even my burr could eat it up. I have one of these left over from having to chew up an E Z out in a bolt that I broke off in a 440 block. It was already pretty worn, but it was too dull from its previous jobs to cut anymore
Broken screw extractor removal tool The Original Rescue Bit
This is something that should have been resolved with the engine block & heads were prepped at the machine shop by my Father-in-law 15 years ago, but that's water under the bridge at this point.