The FP is fine as is, no exceptions. If you get killed because someone in your group decided to go ahead and shoot the FP, and it REALLY bothers you that much you had to sit part of a wave out, then find another group. If it bothers you that because someone enraged an FP and that FP wiped out the squad, find another group. If every time you see an FP coming you turn tail and run away to avoid dieing, stick yo solo mode with invincibility, because dieing is what happens in a first person shooter game. Goes with the territory, or so I thought.
The FP does help force or at the very least encourage people to work as a team. If people don't feel like playing as a team, then the chances of the team succeeding become slimmer and slimmer. It is possible for individuals to take down an FP in Normal mode, even in a server with 6 people. I used to do it all the time using only a few grenades and a couple twin blasts from the hunting shotgun. As for other weapons, the Bullpup is a waste of ammo. You can hose him with several magazines before killing him- a waste of 80-120 rounds, right? A crossbowman is also quite effective, if used in a map where there is much space between the FP and your team, requiring only a handful of shots, though at their cost they could be better spent elsewhere.
The idea that the FP should do what all the other specimens do is rediculous. Almost every other specimen will target you if you wound it or block its path to its original target. If the FP did so as well, we wouldn't be complaining about his random targetting anymore- people would be whining about how there is no variety in the enemy AI. Leave the FP alone, and learn to adapt your individual playing style to suit a game that requires teamwork. A majority of the people complaining about his targetting style are also pointing out how the people they play with fail to work as a team. The game is EASY when you get a full team of people who are willing to work as a team, and its not impossible to find such a thing either.