Just complete chapter 1, obviously.

Aim down with your gravity concussion gun and shoot. Easy hm ?

Just go on, and you will have this.

I guess you already know : go on.

You have only one place to do it. And you just have to shoot on the giant girder with your gravity concussion gun when a zombie is near by.

I was personnally to hurry so I climbed the barrels to get it... But the easier way and the right way to have it and the achievement too it's to use the gravity concussion gun when you will be back there.

Finish the Game.

To Achieve this, you need to zoom up on the rq (with the key you have) and do that for all of them. No need to explain much more.

To do it, just aim the containers just upside you in the last level. Just be sure the strider is right under it, anticipate the movement. It's hard first times, but then it's simple. Place yourself on the first " tower " and wait the strider. Shoot. For the second one, you need to bring him, face to face, under the container. And when it's the right moment, shoot it.

You need to bring the gnome beside the Gravity Concussion Gun to the other one located in a box in the chapter 3. Be sure to keep it with you. The best way is to put it on the floor behind the generator before the Zombie invasion. Throw it away when you have to. Jump and left click to throw the gnome higher.

As written, find all the notes. For some people, you have to come back to the previous checkpoint and get the note again. You will normally have the achievement even if you already took it before. Just do it anew. [ It's an achievement's bug ]

To catch the crossbow, you need to shoot the windows when you entered in the corridor, and you will spot a hole in these windows, just aim the chair blocking the opening door in the back. Then, go on the other side, and open the door, easy !

Here is the OST :