Archimedes said "give me a lever..., and a fulcrum and I shall move the world. Just as a lever would be useless without a fulcrum, hammering without an anvil, or other means of concentrating the force won't get you anywhere. You need to support the crank arm and provide inertial resistance, or all you'll do is mash the pin, and damage the BB bearings.

I don't know what tools you have, but you'll need a piece of 3/4" or 1" steel pipe a few inches long, or a block of hardwood with a hole drilled in the end. You'll also need a heavy vise, or other mass. If you have a helper it'll be easier, or if you have a repair stand you can use that, but your pipe will have to be long enough to reach the floor.

Support the crank arm over the pipe with the large end of the cotter pin inside. Support the other end of the pipe in your vise, or an anvil, or a plate on the floor. Don't put the end of the pipe directly on the floor, it'll score it). Now the goal is to deliver a solid blow to the pin and send it hurtling down the pipe. You don't want repeated smaller blows which will only mushroom the pin, but one devastating shot.

If you've already mushroomed the cotter pin, file off the spread section, and use a punch to drive the cotter out.