Best way to decide whether or not you can kill one depends on if your dino is bigger than it, and if its a carnivore. If both are no, kill with caution. Also, any mid to end game "fliers" will work(wyverns, manas, rock drakes, etc.).
I found a 175 blueprint (acsendant) that was wicked awesome, but it cost 4000 metal ingots, 10000 element dust, 500 oil, 2500 crystal, and 3000 polymer. Am currently farming resources for it, afraid I may never make it.
For a basic enforcer, go around the city and gather at least 500 element dust from lampposts(preferably more), kill a few isolated enforcers until you get 20 oil, go to the wasteland killing the weaker corrupted creatures for corrupted nodules (they can be used as polymer)
When you collect all the resources, go to a city terminal and craft your own enforcer.
To get a blue print of an enforcer your will need to kill one and got to it's inventory and collect the blue print (the higher the level does not mean you could always get a good blue print but I'm not fully sure!) You will need metal ingot,oil,element dust/normal element etc
The Enforcer drops a blueprint upon being killed and the blueprint dropped will give you that exact enforcer you just deafeated. It will have the same color(s) and stats as the one you killed and can be crafted at any city terminal. The higher the stats the more ingredients you will need.