[aland]

I'd suggest a decent set of sharp metal drill bits and progressively drill larger until the wall of the remains of the bolt is thin enough to bend out. A small round file will be useful at this stage also.
This all assumes you can get the first drill reasonably central to start with. A center-punch may help here.

You can get reverse drill-bits which aim to try to encourage the remaining bolt to loosen as you go - if you can get these (and sharp ones!) then even better.

If you can safely get some heat onto the remaining stub it would help - but probably not be a good idea under there!