To get this achievement you will need to finish the 10th level in the game. This level is really short and shouldnt take you that long. You will start the mission crouched behind a wall, you will sww marie to your left crouched down by a window with a sniper shotting thru the window. Sprint past the window and push the wardrobe thingy by using the A button a button command window will open if you fail this you will be sniped. Now just do this ywo more times and marie will go into the basement and you will need to find a shotgun, this is located up the stairs in a wardrobe to the right. If you go to the left you can grab a passport as well. Now leave the house whilst making sure to avoid the sniper fire knocking down the window cover. Once outside make sure keep hold of the sprint button and always try to run left or right if you dont he will tap the sniper on you and you will die almost straight away. Also make sure to avoid the barrles as they take a lot of your health away. Now all you have to do is keep following the path until you get to a stone arch that will put you shortly into a button window again, once agian failure will kill you. Once you pass this you will have to infiltrate the barn this is very easy you just need a short sprint up to the barn and thru the right alley. You will now have your chance to kill the assassin that has been taking cheap shots on you all the time. First kick down both the propane tanks located on the top right and top left side of the barn. now when ever the sniper is close to em shot em to do considearble damage and fill up a small chunk of your adrenalin bar. After you batter him for a while you will jump donw and chase him into a small area of the barn. Now you will have to kill the boss in hand-to-nad in less than 2 minuets for this i would sugest just spamming the X and Y button the boss wont have a real chance to counter and you should beat him easily.