Heres an easy one to program, which is impossible to beat, and completely idiotic. Set the bot's gun to point straight at the player, and when the player comes into sight, kill the player immediately.

Ok, anyway, a decent AI, for an FPS type game in a war situation, needs a bot that reacts like a human. Humans are not always alert and don't always aim straight, are nervous and sometimes react stupidly in war like situations. I think it would be nice to have a bot that becomes afraid when you start shooting at him, or get a freight if you jump out from behind a corner. Obviously, that would be for the more easier difficulties of the game, and they are harder to program. The harder the difficulty, the easier it becomes to program.

EDIT | BTW, in the 3dgs manual they outline a few bot states somewhere, if I remember correctly.