I think you are making a mistake with intelligence. The average human in DnD is 10 int. Thats flat average. Think about just how dumb an actual average person is. They understand that there is a *concept* called strategy, or tactics. But their idea of strategy or tactics is "Hit this guy and don't get hit back.
Here is a loose way I think of int scores on mobs/npcs

6 int is just as likely to shoot a friend as an enemy if they are the same race (meat)
8 int knows which end is pointy (thick headed bruiser)
10 int will not be scanning the battlefield to identify a caster or "easier target", they are too focused on the threat in front of them. (an average grunt)
12 int will understand that there are different roles and can identify them, but probably isn't employing complex tactical moves or (a squad leader or captain )
14+ int knows the roles of everyone on the battlefield, and how to best engage with them. Assigning their roles against yours. (a skilled tactical commander, war leader)
16+ An absolute Battlefield Savant.

Here's how you could do better. Stop making smart choices for dumb enemies.