www.ultimatehandyman.co.uk

Pneumatic breaker............do the concrete and lift it out, then repair.

Borrow a 5" grinder, get a couple of thin discs.
Abrasives discs will even cut hardened steel.

5 mins tops and its gone.

Cobalt Drill bit? Would that be hard enough to drill the bolt heads out? Other option is cutting it with a torch but unless you know what you are doing , I wouldn't recommend it.