Thanks to Slug
Run a hacksaw around the bottom about six inches up creating a line a mm or so deep where you hope it will crack.
OR, an angle grinder will create the same effect and you can cut most of the way around it.
Whack it with your hammer just on this line. The idea is to swing the bottom section to one side so that you can wedge a ball of newspaper in to stop any further debris getting down the drain.
Then work from the top down a section at a time as you say but smashing just above each joint with the hammer. You can often use the pipe itself to lever the lugs off the wall or just smash the fixing lugs off as you go but that way you will have to crowbar the nails out of the wall.
Hire scaffolding if necessary and have someone else [responsible] around but don’t let them stand below you. Make sure if you are working above a door that the door is locked.
Keep nearby windows closed. Keep kids, pets and breakables out of the way. If working off a ladder lash it top and bottom so it doesn’t move.
When you feel the weight of that topmost section of pipe be ready for it, cos cast iron is heavy stuff especially if working off a ladder. You either have to get it onto your shoulder or across your arms for the descent.
Keep the ‘landing’ zone clear, and think Safety at all times.