I just removed a tub drain from a tub that was in place for about 50 years. After many different attempts, here is what it took:
1. Bought a drain removal tool
2. Machined the tool to remove the taper and to fit the drain better.
3. Used a dremmel tool and cut off the top lip of the drain - had to pry off with a screw driver
4. Used my modified tool with a large wrench along with wd40 and broke what was left free and unscrewed.
I had a drain that only had 2 nubs
My recommendation is don't do this - if you do get a dremmel tool or perhaps a 600 lb gorilla