Perhaps you’re talking about the lock ring on a one piece crank (non drive side)? If so, it’s reverse threaded, so make sure you’re turning it the correct way. If it’s really cross threaded, hit it with penetrating oil and use a big wrench. With luck, the damage will
mostly be to the threads on the nut rather than the crank, but even if it’s the crank, it’s not an expensive part.