This will be pretty short.

After I decided to 100% the game. Managed to get all achievements, but one.
The great nemesis enters the ring "Guard Slayer" achievement.
Not that it is very hard or anything. The problem is the randomness of it.

First of all "GUARDIANS" mutation is pretty expensive in terms of power (that is how the game picks mutations) lower ones appear much more often. AND there are other mutations that "block" the ability to get GUARDIANS. They are mutually exclusive these include

• Spitter Guard
  
• Brute Guard
  
• Enraged Guard
  
• Impact
  
• Carapace
  
• Bombardment

I found the optimal way to be:

1. Buy few power ups from the campaign show. Preferably at least mini gunner and a medical pod. The easier it will be. They will be active until you COMPLETE any mission on in the campaign.
   
2. Select The Abyss mission, with easy difficulty.
   
3. Wait few seconds until you get your first mutation. If it is GUARDIANS, bingo you just hit the jackpot. However it is unlikely since its power cost is very high.
   
4. Next thing to look for is "GUARDIANS Mutator"
   
   
5. If you don't see one on the map just abandon and repeat from step 2 until you get it. Your purchased power-ups will still be active.

When you either get it as the first mutation or find it in mutation attached to hive. You should go for it as the very first target. Since if you get any of the incompatible mutations, it will get changed, in that case you can RESTART the mission to get same layout and try again.