That's why there's things called Difficulty. Play at the hardest difficulty if that's the type of gamer you are. And pretty sure that was very early on in the game, as you progress enemies get harder/trickier hence the "weaknesses." Plus the player knew the game and knew what to do to give himself an advantage over each situation. This gameplay showed legendary classic type potential.
Thats just an excuse for bad AI. It doesn't make the AI smarter it just makes their guns do ten times the damage so you die in one shot and your health is easy to reduce. EX: Cod.... Dumb AI through all levels just weapons get damage boosted