Okay, what stage should I start replacing Zombiwittz? Right now, he is my best unit, but I have noticed that his ability to only be placed once is quite a disadvantage too.
I only use the Stuntman because he always gets rid of the Machine Gunner or Sniper before they wreak too much havoc on my tanks. Every time I use him, he rushes ahead, jumps over the barricade, and kills the Sniper, Machine Gunner, or any other annoying unit real quick, and survives with more than half health.
I have taken your advice to remove the Biohazard and I am doing better without it. With that empty slot, I managed to use both the Puddler and Defiler and they are both destructive with a tank in front.
Also, I am working on getting the Titan, but for now, I have to stick with another Colossus.
Now, what should I shelve in order to use the Mortician? I have the Infector because he has saved my life many times. There would be situations where half-way through completing the mission, I would barely have an army or any drones to mutate. Fortunately, I had about 4 or 5 Infectors that created around 20 drones plus another 10-20 so in my reserve. With that amount, I almost had the same amount as when I started the mission, allowing me to slowly pile up my army and overpower them.