Black Mesa: “The Plan” Achievement Guide

First, you need to find the cyanogen.
Starting in the XEN chapter, you will need to progress to and through to the ichthyosaur water-cave.
Here, you find the xen laboratory.
Once you enter the laboratory, progress like normal until you reach the following room.
In the room, you’ll find the cyanogen tank on the table.
Pick up the cyanogen, you will now need to carry it through the island portal at the end of the Xen chapter.

Once you teleport to the gonarch island, the cyanogen should be in front of you, propped up against a rock.
Pick up the cyanogen and carry it to the outside area.You then need to put the cyanogen in a safe area close to but not too close to the portal, as to not lose the cyanogen during the fight.
Once you beat the gonarch, pick up the cyanogen and jump down into the gonarch’s cave, starting the “Gonarch’s Lair” chapter.

In the cave, look around the starting area for this teleporter.

Place the cyanogen on top of the teleporter and press the button, which will teleport it to the end of the “Gonarch’s Lair” chapter.
The button will not appear if you have not done the achievement correctly thus far.
When you teleport the cyanogen, progress through the rest of Gonarch’s lair up until the final battle.
During the final chase before the battle, if you teleported the cyanogen, there will be a hole in the wall which you must go into instead of running to the arena.

Once you go through the hole, you’ll reach the cyanogen machine.
The cyanogen should be placed next to the machine, allowing you to place it inside.
After placing the cyanogen inside the machine, press the button.
The arena below will be filled with gas, killing the gonarch in a few seconds and skipping the fight entirely.
Once gassing the gonarch, you will get the achievement.