It need not act like a human player, it just must help with achieving the goals.
You could also throw in control to allow human players some influence over the AI.
The key thing you must remember when making AI for a campaign is that it need not be fully intelligent. Where as a human like me or you must be able to vary our play between maps and identify and create stratergies on new maps as well as extend our unit choices based on availability, the AI just has to play that specific map.
As the map is pre defined, you can illiminate the need for excessive intelligence and instead resort to telling the AI how to act in a sort of scripted way. Yes variences are still present so some intelligence is needed for low level control but the general behaviour of the AI can be dictated in a very pre determined way.
The key areas needed for an AI are to behave human like in the map. Identify what humans generally do (there must be some colleration between or people who win) and get the AI to try and mimic it. As for teamwork, you could try hard codeing team stratergies into it and allowing them to be chat activated.
Ofcourse the AI need not be able to beat insane, but it could still be able to guide players trhough lower difficulties.
Ultimatly the question does get raised however.... If you want to play the campaign alone, why not play the real thing?
As for your argument about insane AI, that is mostly down to unit composition. The AI has no dynamic unit composition as far as I know, as such it will keep building crap units all game instead of an army to counter you. Blizzard has always had this major flaw with thier AIs, and even in SC2 the AI will rather send weak units into your invincible defenses rather than building a fleet of battlecruisers which could physically bulldoze you in seconds due to lacking AA potential. Even if they have twice your resources and many more expansions than you, they are still limited to the same population cap you are so you just have to expend that more effectivly to win.I know Insane is by far the good AI controllable computer, but still I kicked its ass with better resources and such.
Sorry, I have not.Have you played Wanderers of Sorceria? Are you suggesting something similar to Yue Fang's army ordering to defeat Cinder?
Ultimatly the question does get raised however.... If you want to play the campaign alone, why not play the real thing?