After some serious web searching, I found out that "FC" is the "Reward" for killing something.
What does "FC" stand for? Surprisingly, nobody seems to know for sure, but the best guess I saw was "Frag Coin(s)". Some guesses were on the risque side.
It's just the points you normally get for the amount and variety of the kills.
Used for leader boards and 1 upmanship. changes depending on difficulty and so on.
Other than that it's kind of useless.
As for me the latest trainer matching the patch version works in a fashion.
The max (reset to 1000) health and armour apply to all including grunts and everybody else. They turn greenish.
Usually shoot them with a pistol and dispatch them and like yourself 1 hit returns them to natural full health ( If that makes sense) yes they become harder to kill but you will survive.
As a side fact and because of the trainer;
Melee kills sometimes won't kill the large spider guys.
They may look dead but move close and you can rip their heads off multiple times.
Thanks for the Effort Guys and gals.
Keep up the great work.