At Summer Hall, three armies descended on Robert separately and he defeated each at different times of the day. I just read the Theon sample chapter and I can't help but feel that the exact same thing is happening to Stannis. Robert gained a lot of fame due to his conquering of the three armies at Summer Hall, and he captured two of the three Lords who came against him. Like this, three armies are marching against Stannis as of the beginning of The Winds of Winter. The forces Roose Bolton has sent; Ser Stupid is Hosteen Frey, Lord Too-Fat is Wyman Manderly and the Bastard is Ramsay Bolton. These men are a far-cry from fabled battle commanders, unlike Stannis.
Theon knows these men and claims that each army will descend on Stannis' forces separately -- this is the most important part, because people believe that the Frey and Bolton forces aren't smart enough to combine with one another; and we as readers have the foresight to know where Manderly's intentions truly are. I have a hard time believing that any of these armies will be able to defeat Stannis separately unless they combine their forces, which it seems as though it won't happen. Further more, I always got the impression that when the Manderly's were clashing with the Frey's at Winterfell during ADwD, Roose sent them out to win or die, in a bit of a win-win situation.
Stannis has definitely overcome worse odds and is far too proud to surrender to the likes of Frey or Bolton. I'm trying really hard and I can't imagine him being outsmarted by anyone, let alone Boltons and Freys. He has to win, right? I suppose George has surprised us before.
This all may be just wishful thinking on my part but I also believe that Ramsay's note to Jon was a lie, with a small part of me wanting it to be a trick played by Mance, but I digress. There are just too many similarities to what is happening to Stannis right now to what happened to Robert at Summer Hall during the War of the Usurper for it to go a different way.
P.S. I'm sorry if this comparison has already been made, I just thought of it and figured it was quite interesting.