Mexico could have won in 1836 simply by keeping half their men on duty, with loaded weapons at San Jacinto, instead of the whole lot having that foolish siesta. March being a cool month, there was absolutely no need for it. But I always thought Santa Anna was an A. H. If Houston had been beaten here, I don't think Texas could have succeeded in its quest for independance.
Now, having Mexico win after 1845 is a lot more compex. They would have needed good officers, modern artillery, and a lot more creole and mestizo volunteers in the ranks, as opposed to Indian conscripts.