I live on a hill. Can you practice the underhand cast & be accurate without being on level ground.

Of course. My favorite technique, and by far my most accurate.

 

Edit* I guess I should pause and say that mine might be a mix of under hand and sidearm. Afraid I will get called out by the experts. Lol

As long as you are practicing, learn the "roll" cast off to your side  wherein using the weight of your lure for an extra advantage to your cast.

The hill helps, but you need to be up off the ground.  I suggest standing on something stable such as a picnic table.  I've stood on a 5-gallon bucket before for practicing casts....wouldn't recommend it unless you're as agile as a gymnast.