Call of Juarez: Bound in Blood

The achievement explains it all, completing the game on medium difficulty is quite easy, just take your time when shooting enemies and upgrade your weapons when you can so you save as much ammunition as possible. When you have finally completed the game on medium difficulty you will unlock two achievements, one for easy difficulty and one for medium difficulty.

For an added note you can also work towards a third achievement when completing the medium difficulty, just play as one character all the way through the game and a third achievement will unlock. (even if you cannot play as the character you want due to the character not being available in the level the achievement will still unlock)