You have to do these on the gastric acid (green water of the Apothicon) while the Apothicon is gassy (green fog in the air).

• Kill 50 Zombies
  
• Kill 10 Parasites
  
• Kill 10 Spiders
  - When you stay in the Apothicon for a few rounds, a Spider round will eventually start.
  - Just kill every spider on that round and you're done.
  
• Kill 10 Furies
  - Round 14-16, a Parasite round will start that spawns in Furies at the start.
  - Starting at round 13, Furies will spawn regularly but in small amounts.
  
• Kill 10 Keepers
  - Round 22-24, a Parasite round will start that spawns in Furies and Keepers at the start.
  - Starting at round 13, Keepers will spawn regularly but in small amounts.
  
• Kill 2 Margwas
  - NOTE: You have to oneshot kill the last head of the Margwas. (I suggest use the Thundergun
  only when the last mouth is open)
  - You have to kill 2 different types of Margwas, example: Fire & Regular | Regular & Demon Gate
  | Demon Gate & Fire.

You'll hear a trigger noise like a growl like the one in Gorod Krovi, thats a step completion only.
You'll hear a chime like noise like the one in Gorod Krovi when you complete this.

Many guides said that you have to take the Power Core and/or the Helmet of the Panzer off to complete it, but it's wrong.