Had a friend bring one to me the other day. Shimano Octalink BB and chainset with a seized bolt. Bolt head had been wrecked in their attempts to remove.
It started with me just offering guidance. Ended up with me joining in. Tried everything to get the bugger off. Sawing the axle, cutting two slots and prising and tapping off. In the end after a couple of fruitless hours I gave up and broke out the angle grinder.
10 minutes of careful work later the crank and spider were in 5 pieces lying on the floor. Luckily he was removing the crank to change the groupset including BB – so it didn’t matter that I had to destroy the thing.