What is the best way to level the base of a stand up shower I'm installing.
The back end needs to be raised a 1/2 inch

So you are installing a new shower pan, right? 1/2" is a bit much to hope to raise with just mortar of fill. I would cut some scaps of 1/2" ply wood or backerboard and raise the back edge and then install some strips of 1/4" through the middle. Before setting the pan I'd cover the floor area will well placed baseball sized globs of modified thinset, prevents vreaking and movement later on.