I want to update my master bathroom and would like to know the best method for removing the current cultured marble slab walls from the shower stall.

They are typically only glues to the wall with liquid nails. If you are trying to save the pieces you will need a longer thin pry bar, but good luck. It is so thin that it typically breaks once pressure is applied. I remove a ton of theses any usually take a hammer to the middle of the wall & remove it in pieces,