Presuming it is cast iron not steel (which would be rare) the best way is of course to break the bath and take it downstairs in pieces. NOT EASY but ....
Some folk will tell you that cast iron is brittle (it is) and to just bash the bath with a big hammer. I did that for half an hour with no result at all other than a headache.
I did break it eventually by getting an angle grinder with a metal cutting blade and then I cut slits in the bath rim every 12" or so (just on one side was enough). Makes a nasty mess the dust that is created. Then I put a cold chisel into the slot and belted it HARD with a lump hammer. The bath was split into pieces that way and I could get the bits downstairs - STILL HEAVY though.
Then I took the cast iron bits, the lead waste pipe, the taps etc to the scrap place - and received £35. This was about 4 years ago
5 Precepts of Buddhism seem appropriate. Refrain from taking life. Refrain from taking that which is not given. Refrain from misconduct. Refrain from lying. Refrain from intoxicants which lead to loss of mindfulness