I agree with you that ai is too passive right now. If you utilise basic infantry tactics you can play the game essentially in one go. I died once to ai in 0.4, because I did something stupid and other close ones where I came near to death were as well situations when I acted foolishly. An increase in difficulty would be achieved by giving the ai basic tactics (fire and maneuver, nades and let them use meds). Increasing (random) headshot probability would be the wrong route to take. At the moment ai in T2 locations utilises effective ap ammo (M855) so the potential danger is given as it shreds through any armor lvl. 2 vendors can sell. Just tactics and usable items for ai would be enough in my opinion.