The thing with the AIs is that they didn't make them totally impossible. I have seen them -on numerous occasions, have their ACU just sit and do nothing for minutes on end. And if that's not enough, they rarely move their ACU (intelligently) away from danger. If he's got a task in mind he'll just keep going back into danger.... Idiot. :D
I can see situations in which it could have done a very smart thing but passes up opportunities, an AI will "never" have the ability to make rational decisions and all that good stuff.
But as for beating a hard level AI in SupCom, I would first make sure I have a suitable defensive line (AA, PD, shields, etc.) then,when you're sure that you can defend yourself from the inevitable attack (easier said than done...) then you can plan an attack.
Turtling can be key.
I like to play against horde AIs on 5x5 maps such as Winter Duel. If you expand fast enough and put pressure on your opponent, soon enough the ACU will come out to construct defenses or something and, easy win. (relatively speaking, of course.)
SupCom does take a bit of the multi-tasking out of it with the ability to queue commands, so that shortens the gap between the AI and player.