It seems the more damage you do to one, the better the blueprint.
I have killed several so far and got the blueprint with their level, but during one encounter I was attacked by a spider as well, and panicked a bit. The blueprint was for level 37 Enforcer, and the one I fought was level 40. Not quite sure how it's counted, but when I have been fighting two at once (30 and 70) I got both blueprints with level of the creature.
When you have an Enforcer and level him up he can carry quite some weight, has a lot of health (though you shoulden't try to kill anything higher than a corrupted Rex),and he can do a lot of damage. You can even build your fort up high and jump down with him 'cause he has no fall damage.