It does help if a scenario designer makes it clear in the description section what modes the scenario should be played in.....Getting the AI to put up a respectable fight often (but not always) requires the use of forces that would be rather overwhelming in the hands of a human player.
The case @chuckdyke describes above might actually be one of the instances where the AI might perform reasonably well on either side, it's good on the defence and not too bad in overwhelming attacks.