Intelligent monsters will act intelligently and go after the most immediate threats to their safety first , they will be worried about the archers and mages in the back line , and they will try to coordinate their group to keep the melee enemies engaged with their melee units while their ranged units attack their enemies ranged units. Or they will use ambushes to deal with these threats first.

Animal monsters will simply engage the closest enemy and attack and retreat if they are injured.

I will always like the C64 version best because that is the system I had in the 80s and 90s