I finally managed to pop this achievement today after about 5 tries on casual, wasting 2 instakills, partly not sure what to do and what counted, what did not. As mentioned above, all the times prior to popping it, I played on casual. Today I did a test run for the easter egg to practice some stuff and managed to pop the achievement, even though I did not fully commit to going for it. Let me give you an explanation of what the achievement requires you to do:
- You will have to kill either gas, water, fire or electric zombies by exploding them. This explosion can kill nearby zombies as well. You have to kill 9 zombies by causing one of the 4 elemental zombies to explode. This does not have to be done at once, although you must do so in a single game. E.g. you can kill 5 fire zombies with brain rot, each time taking down 1 additional zombie and kill 2 gas zombies with cryofreeze taking down 1 and 3 zombies with the explosion.
- You will need to repack your weapons in order to be able to cause a Catalytic explosion. Each repack can cause a different zombie to explode via a catalytic explosion.
Fire zombies ->> Brain Rot
Water zombies ->> Killowatt
Gas zombies ->> Cryofreeze
Electrical zombies ->> Fire bomb
I personally find brain rot the easiest, as I could clearly see a white bubble appear when the catalytic occurred.
Note: The elemental zombies can explode with any weapon, though note that the achievement requires CATALYTIC explosions and not any explosion caused by these zombies.