There are two locations possible for unlocking this, although, due to the relative scarcity of flamethrower fuel and the tendancy of enemies to be 'bullet sponges' on lower difficulties, one location is much more practical. The most obvious location is the finale; after activating the switch, an endless wave of Spiderbugs will spawn in the enclosed arena - if you have saved all the fuel found throught the mission, it is possible (assuming the player burns all the webs away first) to run around in a large circle, turning around occasionally and burning the pursuers. It is recommended to play on Ranger Mode to ensure that 20 Spiderbugs can be killed with the available fuel - on lower difficulties, it simply takes too much ammuntion to kill one.
An easier and safer way to earn this achievement/trophy is to, similarly, stock up on fuel, and when you are about half-way through the level (after you reach the first switch that opens the doors to the empty silo) burn the spider nest to make the Spiderbugs drop down and kill all of the attackers that spawn with the flashlight and Ashot. Climb up one ladder to the next identical level. If you look on the walls, there are several holes and every few seconds a Spiderbug will exit one, run along the wall and disappear into another. These Spiderbugs spawn endlessly, and as long as you stand closest to the edges of the drop (note that there is an invisible barrier to prevent you falling to your death) then the Spiderbugs will be unable to hurt you. Regardless of difficulty these 'ambient' Spiderbugs seem to die from minimal amounts of damage, so you should have more than enough fuel to unlock this achievement.
It is also recommended, with either strategy, that the player turn their flashlight off to prevent any kills being 'stolen' through light exposure, as opposed to flame damage.
- Interestingly, the word "Arachnophobia", in medical terms, mean the fear of spiders and other arachids like scorpions. This is ironic, considering you are burning many of them.
- The trophy name "Arachnophobia" is most likely a reference to the movie of the same name in which an infestation of deadly hybrid spiders are eventually killed by lighting them on fire.