I would attack the shackle. As far from the bike frame as I could, on an angle that won't send the shower sparks directly onto the bike. I would also wear eye protection, and not hold my face in line with the wheel. I would figure out a way to hold the lock securely so that I could have both hands on the grinder. If the shower of sparks lands on your clothes it will eventually burn a hole in them. Don't worry, you'll know it when that happens.