I had a similar experience with a recent stubborn cotter. I resorted to drilling, but got careless in the process and managed to snap off the tip of a drill bit in the fresh hole I had just created! I figured that material was likely harder than the cotter itself. At that point, I had drilled enough of the cotter out so that it was a few mm below the crank arm. I was able to fit a small socket from a wrench set in that hole and than used the cotter press to apply force. That sucker popped right out.