The achievement will unlock after you finished Act 8 on Assassin and will stack with the Easy achievement. Let's review a little walkthrough:
1 Explosive Rat
9 Concept Arts
The section we start off in isn't too hard. You will have 2 enemies come from the area where we need to go next and a three off to the barricades on the left. When you head down into the sewers a knife enemy will charge you after you turn your first corner. Watch out for the 3 mine sequence as you move through the sewers. When you get to the end of the enclosed sewers, as soon as you go out an upgraded knife enemy will appear and will charge. As soon as you go to move on you will hit a checkpoint and a mine, destroy it to move on.
(Explosive Rat found in this first checkpoint area).
The first area you walk up into will contain a shoot off. As soon as you move beyond your initial cover more enemies will spawn most notably a shot-gunner. When you move into the next courtyard it will be a larger scenario of shooting. There is a concept art in this area (the restaurant setting to the right of your original entrance to the area). After passing through the courtyard you will need to go back down into another sewer system. There will be another concept art down inside to the left. Once you move close to where the mine is you will hit a checkpoint.
(2 concept arts found in this checkpoint).
When proceeding through the sewers you will run into another upgraded knife enemy. You will eventually head out into an open area with an overturned boat. Behind the boat is a picture. Defeat a few enemies including one in the window up and to your left from the boat perspective. When you go up the stairs and head into the next area prepare for another fight. In the back right hand corner of this area will be another concept art. Head through the hallways until you hit a cutscene and the next portion of the map.
(1 picture 1 concept art found in this checkpoint).
As soon as the cutscene is finished turn around and head all the way back to the gate. You will find the poster off to the right here. During the sniper sequence you will need to enter a house on your right, but before going in check for a concept art to the left of the stairs used to enter. Upon entering there will be an enemy behind the cover in your path. When you emerge from the building the sniper will have move on allowing you to enter that building.
(1 Poster, 1 Concept Art found in this checkpoint).
At the top of it, in front of the blocked off staircase will be another concept art. Be careful though, when you initially go up the staircase a knife enemy will come down the hallway as well as 2 gunners. After you defeat the gunners beware of another knife enemy that will run all the way from the far sniper building. I suggest waiting until he comes down the hallway to avoid being surprised and killed by him. Now you simply need to take your time and work through the enemies being sure to stay in cover from the sniper. You should enter the house to your right to avoid the sniper and work around the mines and open enemies. Upon entering you reach another checkpoint.
(1 Concept Art found in this checkpoint).
Going to the doorway to exit the building will spawn a knife enemy so as soon as you go outside, dart back inside and wait for him. Once he is gone slowly work your way through the remaining gunners. There will be quite a bit of gunfire so definitely take your time and work through cover. Entering the house will net you another checkpoint.
At the bottom of the staircase is another concept art there will only be one enemy down here. Once you get upstairs you will need to get to the other side of the room to go up the other set of stairs. In order to do this you will need to go through quite a few enemies. At the top of the stairs will be a checkpoint, and a cutscene.
(1 Concept Art found in this checkpoint).
The key with the sniper is to take your time. There is no time, and the enemies cannot shoot through your cover so just focus on one enemy, kill him, and then focus on another. Slowly work your way through this.
As soon as you emerge from the sniper scene and you move forward you will be attacked by a knife enemy. Dispatch him and begin to work your way into the next area. It is heavily defended as well as occupied by a sniper. After you kill the sniper and move onto the bridge you will unlock the next checkpoint.
Moving on the bridge will spawn a knife enemy as well as 2 gunners on either side of the exit to the bridge. Once you pass the rubble for cover to your left will be a dumpster with a concept art behind it to the left. Pushing through this next area will again be a slow task as your enemies are dug in and will toss grenades. To the left of the mines will be another concept art. Heading up the stairs will grab you another checkpoint.
(2 Concept Art found in this checkpoint).
To the left of the sandbags at the tops of the stairs will be a picture under one of the benches to the left. You need to go into cover to get it. This is the final area. It certainly will be the toughest part as the sniper will keep you pinned down and the number of enemies will be high. Just remember to use your explosive bullets and clear a path to rid yourself of the sniper. Before going through the exit head all the way to the right to grab another concept art behind the boxes.
(1 Picture, 1 Concept Art found in this checkpoint).
I hope you enjoyed this detailed walkthrough, and I hope it helps you during this tough mission.