This achievement can be easily done during the MP training mission.
The first target you are given during this mission is walking circles around a few buildings. In the process of doing so, he also walks by 2 hay stacks. Jump into one of these.
First, wait until the circle around the X button prompt completely fills and you hear a tone. Then, wait for the target to turn his back towards you a bit so you attack him from behind. Assassinating him like this should result in exactly 750 points, unlocking your achievement. If for some reason it doesn't give you a score of 750 on your first try (it gave me 650 on my first try, but I attacked him from the front), you can easily retry the mission until it does unlock.
If you didn't have the haystack achievement before attempting this, you will also earn that achievement.