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.

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