You need to complete a board on each difficulty.
You can do this legit of course but if you suck at this game on medium and hard like me....
play a board and trigger a mine, then select view board, then make a screenshot you can do this with another camera/phone or on your laptop to work fast.
I did this on my phone: press the home (windows) button and the on button simultaniously and your phone will make a screenshot.
Edit: with the newer phone updates: press the power and volume up button simultaneously to make a screenshot.
Select replay board, then push the home button and open your screenshot (pictures-->albums-->screenshots) now use your <-- button (hold it) to use it as a taskmanager switching between the game and your screenshot.
this way you can finish medium and hard difficulty and unlock some medals at the same time (finish medium diff without using flags).
edit: uploading/sending your screenshot to another PC/Phone/Tablet will make things easier since you don't have to switch between your game and the screenshot.
as Christian Doe states, it might also unlock with a possible glitch see comments. Let me know if this works for you as well.