I've made my way through everything off-line multiple times at various levels (including a level 20) and just don't get what's so hard about the game. The team A.I. is weak, absolutely, but if they were any smarter the game would become quite easy.
Why then do people want things nerfed, or maps changed? Do people want the game to be easy? If that's the case why not just play on easy?
The problem, as many people have stated, is not that it is challenging. Whether you win or lose is luck. You cannot control your NPC teammates or stop them from being idiots. You similarly cannot defeat the enemy team by yourself, regardless of how skilled you are. If I switch to a class I need to be to defend or assault a point, then the NPC's who are the appropriate class to interact with an object simply will not do so. If I attempt to fight NPC's, they will headshot me at ridiculous distances and kill me in the blink of an eye. Every enemy NPC focuses on me to a ridiculous extent. Allied NPC's do not do their job.
A Challenging game tests my skill. This game does not test my skill. I have played a long list of games- Atari 2600, NES, PSone, N64, Dreamcast, PS2, Gamecube, Xbox, Xbox 360, Ps3, and of course PC. Most games I have played challenged my skill. Only a few forced me to win or lose based on factors beyond my control. This game is one of them.
I have played a long list of games. I have never encountered missions as unwinnable as those in the singleplayer of this game. It's possible that the difficulty of singleplayer increases the more you play singleplayer, rather than based on rank or level. If that is the case, then you might need to level a character from 1-20 in singleplayer, and then play a few singleplayer missions afterward, to see the whole effect.