Using self leveling cement in basement bathroom around drain
Old question but to give a correct answer... You don't level basement floors - PERIOD.   

You have two options:


Add a subfloor and shim that to flat - dricore squares are an example.   The problem with this is not the bathroom, it is the bathroom floor will be 2" higher that the rest of your basement.
Buy flooring that you can install over this.   I have installed many many basement bathrooms that had slanted floors (they all do).  I have used pure vinyl (waterproof not water resistant) planks and they for sure bend enough as long as the floor is pretty flat.  The planks are very high end and can look however you want.   


This is a very typical basement bathroom.   The water in your basement must be able to drain.   It isn't just for the bathroom but it could be sloped for a good part of the basement to here.   You don't get to just "level" it.