IF you mushroomed the cotters, you are pretty much screwed.
This is why cottered cranks fell out of favor years ago. They suck bad!
Next time, back the nut off just to the end of the bolt and then whack the bolt. This will help keep you from mushrooming the bolt. There is a device that some old timer bike shops have to press the bolt out, but if you mushroomed the bolt already, there is nothing they can do.
Drilling out the bolt is nearly impossible without a machine shop because you have to drill perfectly straight which is very hard to do with a hand drill on a bicycle. Remember, the spindle is hardened. If you drill and damage the spindle, you are double screwed.
Be sure to support under the bolt. The whacking can damage the bearing cups. I broke some bearing cups trying to remove the crank arms once.
The rule of thumb is to:
1) make sure you have good support for the crank under the bolt before you start whacking.
2) Use a hardened pin to hit the bolt. Do not hit the hammer directly to the bolt. I use the spindle from an old pedal for this.
3) Use liquid wrench
4) tap the crank with the hammer in the opposite direction of the crank rotation and a little on each side of the crank to loosen it up a bit.
5) When you do whack, do it with gusto. It will come free only with a strong whack, but the light and medium hits only mushroom the bolt.
6) Maximum two or MAYBE three whacks and it is time to give up and bring it to an LBS that has a bolt remover. Don't mushroom the bolt!