Of all the scenarios, Wading into Battle was a tough nut to crack. 

I've posted a play-by-play guide (w/ screen shots) toward achieving the same result on steam (avalible here).

https://steamcommunity.com/sharedfiles/filedetails/?id=2190896765

Quick take-aways: 

- Line of Sight (LoS) and vision are key

- The AI has Initative, engages first, but will not try to ford the river (unless baited or forced).

- Rough terrain limits ranged units (not siege) greatly. 

- AI starts turns with longest ranged to closest range.

Since the AI uses its longest range first, it'll position them forward to attack "bait" before melee units can fill-in the front line (trapping ranged units behind them). Picking off the AI ranged units helps immensly (2-3 on the left side & cannon on the right). 

Let me know how it plays out in the comments!