Sheared lower bolt on alternator

I was completing a routine alternator swap on my '95 Defender 110 when the lower mounting bolt sheared off. I've tried multiple methods from vice grips to welding a nut on the stump and none have worked. Before I remove the entire mounting bracket to try and drill it out, I thought I'd check here for any other ideas. Anyone have some recommendations? Thanks

Irwin removers are very good.

Is that threaded or a continous hole?

threaded. I have all the sizes of Irwin removers and they stopped gripping it...just kept going round and round.

I would get on bench use something like PB Blaster with it vertical overnight , file some flats on it and wiggle it back and forth with more pb blaster and or heat

Thanks for the replies. I'd better get to work!