Take out the ones with the AK's who are shooting innocent people. What's difficult about that?
Making a 1000 yard cold bore shot at a 10" plate is simple too. You just check your dope for similar conditions, dial your elevation, check the wind, dial or hold over for that, squeeze the trigger and get the hit. Simple.
Combat is not as simple as your terms would make it seem. What is your goal? Where are all the enemies? What are the innocent bystanders doing? On and on. Processing all of this through the reptile brain while going through an adrenaline dump is not particularly easy. Neither is making hits on a two way range.
Yes, it is not calculus. But simple and difficult are not mutually exclusive.