when this happens at work I've been able to use carbide burrs and grind it out.. takes some time but usually works. good luck!
Maybe you can use a center punch to slowly break it up. Be sure to protect you eye balls.
I usually try one of those two suggestions.
Like said, they can take some time, but there's not many other options.
When first drilling a hole for the remover, I try to find a proper sized lefthanded drill bit. Sometimes
that works to get enough of a hold on the broken bolt to remove it and I don't have to try the remover.
You have to watch out with those removers. Too small and they snap easily. Too large and the more you try to turn counter-clockwise, the more it expands the bolt in the bolt hole.
Is it at all possible to heat the area around the broken bolt?