A few words of warning if you're going after this: Make sure you don't start another game (even in another slot), change players or switch your existing game between solo and co-op play until this is awarded. A friend and I went through the entire game on Contractor (which was very frustrating, particularly towards the end) and while the Softcore achievement for the halfway mark did unlock, this didn't. At one point, I started another save to try out different guns and this is what we theorise broke the achievement. The game will not tell you or warn you ahead of time that these things will break the achievement.
UPDATE: The game does in fact remember what checkpoints you have completed on Contractor if you start levels over again. I started playing from the halfway point in single player and midway during the second last level, the achievement popped. Unfortunately, the game won't tell you which checkpoints you're missing but if you have the Softcore achievement, you can start from the zoo mission. Wifelikedrip also pointed out that if you make only negative moral choices in the game, it apparently makes it easier, though I haven't tested this myself.