sawzall with a diamond blade to get started, better yet a cutoff saw with a metal blade. sawzall can take up to a half hour and three or four blades. cutoff saw or grinder with a metal blade ---about A  minute to cut, lots of dust, wear a mask or you'll be coughing and your lungs will have a burning sensation. expect a metallic taste in your mouth if you use a cutoff saw.(from the dust) wear eye protection, litlle shards of the cast iron can lodge into your eye. I speak from experience as a master plumber who has worked with cast iron for years.