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You can find the cyanogen bottle in the lab part of the Chapter 15 - Xen. It is placed on a table with two blue glowsticks, right next to a torn up part of the shelter leading to a cave - the place where you go in order to progress.

After the first encounter with the Gonarch, you will fall into a cave where you will have to blow up a wall. Going further you will find a circular platform with a small panel (the panel is identical to the ones used in many lifts throughout the game). After placing the bottle on the platform you will notice the buttons changing color from red to green. Press the button in order to teleport the bottle to an area above the arena where you kill the Gonarch.

Later in the chapter, during the escape sequence at the end of which you will jump into the arena mentioned before, you will encounter healing items on the ground. Since you've teleported the bottle, a new path will open to the left of those items (normally its blocked by fallen rocks).

There you will have a view of the arena with the Gonarch inside it, the bottle you teleported, standing on a little platform and a big boiler connected to a conveniently gas-bottle-shaped spot.

You will get the achievement after killing the gonarch by filling the arena with gas (press the button on the boiler)