This achievement can be easily done during the MP training mission and will likely also result in gaining you the Overachiever achievement.
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. Simply wait for your target to walk by and hit x to assassinate him. Achievement unlocked.
If you do wish to also pick up the Overachiever achievement, 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 and your second achievement.