All you need is a spy like Moghedien relaying all your oppositions' troop movements to you. Fairly simple to counter what you know is coming before it gets there. He also (supposedly) used a circle of 72 so he would have a huge advantage over Rand if he showed up, effectively wasting channelers that could have been used elsewhere. So the answer to how good a general he was is unanswered IMO, especially given the fact that he had an overwhelming force ( 10:1 ) of Trollocs at his disposal, and still lost the field.

As far as I'm concerned, Mat showed himself to be the far superior general. He outwitted, outmaneuvired Demandred, plus figured out early on to change his strategy for fear of betrayal, and also realized that there was a spy in the command tent intercepting his commands. The deck was stacked against Mat, the only real advantage he had was being a hero of the story and destined to win despite the odds.