What this reminds me of more than anything is the AI in Halo. You have a number of enemy types with comparatively simple behaviour rules, and if the game featured any one of them alone it'd be very dull indeed, like a game of Pacman where all the ghosts behave like Blinky. But the enemies are always presented in mixed groups - an Elite, several grunts, a couple of Jackals - and there's great synergy in the combination. If you can land a plasma shot on the Elite then you can take him down quickly and the group will disperse, but there are grunts firing on you and lobbing grenades. Grunts are an easy kill if you can get them in your sights, but the Elite is pounding you with plasma whenever you break cover - and the jackals' otherwise laughable defensive creep is turning into a serious flanking situation.
That's on Legendary, I'm obliged by law to specify.