I don't think that I will be able to have my painter get the masterpiece challenge without one of the others first (on accident).
This is the easiest challenges to complete, actually...if you know how to do it. All it requires is a painting level high enough to get masterpieces and knowing what the masterpiece paintings will look like when they're done.
Use medium or large paintings, since it takes longer to paint them and you'll be able to tell what they'll look like sooner. Start the painting and only paint long enough to see what it's going to look like when it's finished (it shouldn't take very long...usually during the starting stage). If it's not one of the masterpieces, you scrap it and start over. If it is one of the masterpieces, you finish it.
You don't get credit for scrapped paintings since you didn't finish them. You won't have to worry about accidentally getting a regular or brilliant since all of the masterpieces will use specific images that the lower quality ones won't. You'll also save
a lot of time, since you're only finishing the ones you're looking for.
This is how I always complete this challenge. It's also worth noting that this will not work for Brilliant paintings, since they use the same images that the regular ones do.