Hello and thanks for the comments and donation!
For the army battle, the game uses the same battle system for the regular battles, but with changed damage formulae so that armies could use them. The main difference is a simple switch that only allows you to select heroes in regular battles and armies in army battles.
Armies behave mostly like heroes programming-wise, and there are 20 slots in the player character roster to create them. Their level attribute is used as the troop number, and their damage dealt, healing done and DoT received is multiplied by that number in calculations (For a very simple example, if an army with 10 troops and 8 strength hits an army with 2 defense, they deal 60 damage before factoring weaknesses, resistances and positioning). I also edited the methods that govern taking damage and healing so that if used on an army they deduct from the number of troops or replenish them up to the number you had at the beginning of the battle, respectively.
For everything else I set up a small database that determines what class of armies each Troop item turns into (such as Militia item turning into troops for a Militia army) and the rest is mostly done via the events you access at the barracks.
The main plugin I used is GubiD's Tactical Battle System, which was unfortunately incomplete in its VX Ace version, so I had to patch it up with my own additions, such as differently shaped area attacks, terrain effects etcetera.
Other scripts I've used include Hime's map screenshot (for the terrain window), NeonBlack's large sprite fix, UI and message scripts such as Yanfly and Galv's, and Szyu's crafting system, though I tweaked most of them to adapt to the game's needs (such as adding shortcuts to craft in bulk).