how do i replace my tubdrain and seal
This same thing happened to me and I don't have access underneath....unless I cut into the ceiling from below.
I had a wrench similar to the popular plumbers style channel locks. This one however had jaws with sharp edges on the opposite side of the jaw/teeth .......not rounded.
I was able to wedge the pliers head into the drain opening and open the jaws enough while turning to get enough of a bite on the drain to unthread. It wasn't easy and took many tries and some effort.
Otherwise I have heard of cutting a notch into the top diameter and using a screwdriver or prybar and trying to knock it counter clockwise. Not my first choice but something to consider.
Or there are the tools already mentioned which look like they would work awesome.