GSG Combat Mechanics Tutorials

I've been struggling to find any tutorials or references for how best to implement a good grand strategy combat system not unlike one may see in CK2, EU4, Vicky 2, etc... though I'm attempting for more tactical depth. Imagine CK2 game with unit cards placed into a battle line for combat against an enemy army. Imperator Rome sort of does this?

In detail, basically when 2 opposing forces share the same square tile, a battle begins and the makeup of the army, units, etc... is used for the combat. I'm trying to create it where the each army has a battle line that can be customized beforehand so that opposing units that are across from each other battle it out first, and then on to adjacent units to add some tactical depth to the game.

Does anyone have some ideas on where to start on this? A single unit battle is straight forward (Ie Risk) but the battle line mechanic I'm attempting to do to create tactical depth is creating a writers block and I can't seem to find a jumping off point on Reddit & Unity forums. To clarify, there is no transition to a battle map, it all takes place on the world map similar to how Paradox's GSG's works.

Thanks for your thoughts everyone.