You will hear very loudly to dig out the high side. Yes, you should do this and not build up. However if you must do this it is possible. Maybe just as much work and more expensive though. If you build up, sand will not work. You will need to use something like stonedust (crushed granite or limestone). It also has other names. If that is not available, hardpack will work too (this is the stuff used under driveways / roads.
Depending on your grade you will need to extend the grade out so that the weight and slope are enough to A) handle the load and B) provide proper drainage. May not seem like a lot of material at 1st but it will once you start putting it down both in labor and cost. Put it down a few inches at a time, packing it down with a commercial grade plate compactor. Some will do more than a few inches (bigger and more $ to rent though).
Depending on how deep you need to dig and how many rocks that might be better to dig down.
As for leveling, you can do it various ways. A simple water level for a 15 foot pool is your best bet. Surveyors transit or self leveling laser level being a good rental if you have the $. You can make a water level out of a pc of clear plastic hose and just fill it with water, spike the ground with stakes and measure to a level line from top down.