How to get enough leverage to remove pedals when their cranks are not attached to a bike?
The best option would be to place the crank in a table vise - or in a pipe that has been gripped in a table vise.

If you don't have access to a vise, another option is to try and attach the pedal wrench so that the handle is only a few degrees away from inline with (right on top of) the crank, so that you squeeze the wrench and the crank together.