It seems the AI is very good already, it could use a little more animation (basically it's only moving around walking, the stiff movement needs work and I would add some different ways of walking around). The way they act in gameplay is very good. You've got the basics perfect, I'd say balance is pretty good already too, now all you've got to do is tweak things and add things to make it more believable.

Just add some inbetween actions, some more animations and you're fully set to go, this would make them look less 'bot'-like. Although I do understand people think the AI is looking dumb, it's not about making them smart, it's about making them believable. This is something you fake off course. In this case, their actual behavior is already really good, I'd just add some more animations, perhaps some ducking behind obstacles, but that's about it.

In the end they should be challenging, not too smart per say.

Cheers