The AI is actually somewhat clever and it knows it has an advantage at the end of the map. You'll need to bait them out of their vantage point somehow. The game uses a lot of the teachings of Sun Tzu's Art of War and the battle AI makes a point of utilising the strategies in the texts.
Also I highly recommend NOT playing the game modded until you've actually got a feel for the base game first. Once you've understood what it's all about and how it works, then would be a good time to try any mods if you so wish. Try it as the devs intended, then try the alternatives and see which you prefer as a result.