Most everyone agrees that the strategy of attacking "the squishies" first in a combat scenario is generally the best idea, except in some specific exceptions. Often, then, the decision on whether or not an NPC is "smart enough" to do this comes down to their intelligence score and cultural upbringing - animals are not smart enough to do this, human opponents are, and orcs may or may not be.
Is this an accurate view on intelligence and strategic planning? It seems possible that this kind of strategic thinking is something that we take for granted in the modern day. Being exposed to tons of video games, roleplaying games, and a wealth of historical records may bias us. Is it possible that people only recognize this kind of strategy as being "obvious" due to massive amount of role-based games and combat situations people see these days? How likely are characters in a fantasy environment with less exposure to games, education, and strategic thinking to know how to properly prioritize characters of different offensive/defensive roles? How common is basic strategic understanding to the uneducated?