they have a special snap cutter you can rent, but if you just have one cut to make probably the grinder will work best, try to cut a grove all the way around it, and it should break on the line, some of the old cast iron is only 24 inch sections, so if you can just continue down the pipe till you find where it connects to the next pipe and bust out the whole 24 inch section, good luck

You can use Sawzall with "Grit Iron" blades, "Snap Cutters", or "Chain Cutters". Your cheapest way out of it would be either to rent a pair of snap or chain cutters, or buy the grit iron Sawzall blades

Reciprocating saw with a Bi-Metal blade. Wrap tape around the pipe to mark the cut. Circular saw does not have the depth of cut.

i suggest using a sawzall, u must use carbide tipped blades designed for cast iron, this will make your job a lot easier than bi-metal blades or such. roughly 7.00 or so for one at any hardware store. otherwise, you can rent a chaincutter designed to snap cast iron.

I was in your exact situation a few weeks ago. I didn't realize I was cutting cast iron or how hard it is. I got some good quality blades and started cutting with a reciprocating saw. The first blades were DeWalt, I thought they were faulty and returned them. I then bought Lennox and they cut but very slowly. It took about 15/20 minutes to cut through a 3 inch diameter pipe. Still faster than going to rent a special tool. I also tried a carbide blade for the recipro. saw, not good. Be sure you get the long Lennox blades.

if you don't want to rent snap cutters or cutoff saw use the grinder, just be careful! cast is brittle if you cut across the top and down the sides but cant get the very bottom, a few taps with a hammer will break it off the rest of the way