Well, you better get used to the idea, why else do you think RJ merged in Mat a juvenile trickster, with unnatural luck, with a walking encyclopedia of 2000 years of military knowledge if not to provide Demandred with a worthy foe in the LB?

You overestimate Demandred. The War of Shadow lasted just 10 years. In that time, Demandred first studied warfare in old books and build up armies of people who had no idea how to fight in a war from the ground up, all this to face Forsaken with nearly zero knowledge of warfare themselves who relied on armies of millions of Shadowspawn to face an unprepared, unknowledgeable foe...the early years of the WOS musn't have been fancy, it must have been a complete barbaric and chaotic mess... until Bel Medar/LTT got organized and started using real strategy and tactics against a foe that used primitive ones, relying of sheer numbers and savagery. It's hardly the greatest military minds of all ages Barid Bel Medar faced in his few years serving the Light, and then by mid-war he switched to leading billions of Shadowspawn against leaders of the Light who had by then a few years of field experience and who knew Sammael's and Demandred's styles very well, but were seeing their high tech gizmos disappear by the day, forcing them to rely more and more on channellers. Demandred's certainly very intelligent (far more than Mat), but he's still a gifted dilettante with a few years or real experience when it comes to war. He probably relied a lot on all the classic strategies and tactics he found in books, because the foes he faced didn't know any of that stuff, so all those old tricks worked. He won't have it so easy with Mat, who knows all this old stuff, and new stuff developped over 3000 years (and again, we've got the clues Demandred has started learning about all this in books)... Don't forget, Mat has greater "experience" (memories) of fighting Shadowspawn armies than Demandred and LTT combined. Ishamael isn't a military mind (though Moridin commented once he has studied since the WOS, even in fields favoured by Sammael, suggesting he's not hapless in warfare nowadays), he probably relied heavily on strategies developped by the Shadow's generals in the WOS for the Trolloc Wars, and Mat has a whole compendium of memories from that time.

Demandred's great advantage over Mat is that he's got the initiative and a whole lot of time to prepare his campaign, while Mat will have to improvise. But in the ten years of WOS, he'll have developped a personal style of generalling - again his luck may be that Mat won't have access to any of Rand's knowledge of Demandred's style for a while yet and will have to figure him out all by himself. But RJ has made Mat the ultimate general, because he doesn't have personal habits and quirks, Mat's own military knowledge is a total blank slate - whatever seems best in any given situation rather comes to him from his memories. He fights like one general one hour, and like another the next... extremely unpredictable. Not easy for someone even as intelligent as Demandred to fight a foe who switch pattern every day. You think you've figured out Mat, and he still does something unexpected.

My feeling is that Mat in the first part of the book will finally have a real challenge with Demandred and the Seanchan army may never be called "ever victorious" again. However, later in the war when Mat has had time to figure out Demandred, he's likely to turn the tide and ultimately defeat him.

Don't expect Rand will lead the campaign against Demandred, he flatly said in TOM he doesn't intend to lead the war this time around. It doesn't mean he won't finish him up ultimately, but he's not the one who will lead the campaign against him. And it's a boon for the Light, because all of Rand's battle knowledge comes from LTT and Demandred knows LTT all too well, and has no doubt prepared himself to fight a LTT who can rely only on third Age resources but for channelers he can train to use more advanced stuff. Demandred believes he knows his foe very well, he's in for a bad surprise. He'll sweat too, not only Mat will.