I can do regular finishers no problem but for some reason it just doesn’t seem to work when I try a stage finish. What is varies suppose to mean? How do you do a stage finish 100% of the time?
iirc you can’t do a stage fatality in the living forest because it wasn’t included in the demo. But I think you can do it in the pit.
Varies means that the stage fatality varies by stage. example: Living Forest = Throw guy in tree. Pit = whatever the pit fatality is.
to do it 100% of the time, you input the motion required to do a stage fatality 
on the stage select screen, every stage with a stage fat will have a red skull icon on it.
also, i am pretty sure the Taxi cab thing from the demo montage is a stage fat as well for the Streets stage