Since the op started with Rome total war, I'm going to give an example from it too.
I was (re)taking a Greek city that had just rebelled for the 19th fucking time in 20 turns. This time I wasn't going to take any shit from them, and I would garrison a full army.
Comes the day of the battle, they come out of the castle to meet me in an open field right before I can besiege it.
Day of the battle: a pleasant summer's night in a Greek prairie.
I'm fielding a classic composition of legionnaires, archers, some praetorian cavalry and a few arbalests.
Another army is coming as reinforcement, with fire piggies ( pigs dosed with oil, set on fire and set into enemy lines), with javelin auxiliaries and a catapult.
Against me is a HORDE of random units. Most of them very powerful melee units.
I start moving in, close enough for my javelins to have just enough time to fire off a couple missiles before retreating behind my defensive line. Then I move in my fire piggies, and let my archers open fire. The enemy understandingly doesn't appreciate and rushes in. I sic my piggies on them, which would allow my archers to fire at routing, confused, hurt units.
The piggies run forward....forward.... then they decide to have some revenge on me and double back.
I swear, those fucking pigs went through my ENTIRE line, zigzagging for maximum carnage. My javelins? Still busy running every which way and get trampled by barbarians. My legionnaires? Still reeling from having flaming pigs running into their junks at mach 2, are decimated in seconds. Archers as squishy as hell and could barely do anything, and my artillery that was SUPPOSED to fire at routing targets, fired right into the fray, killing probably more of what little left there were of my men, than my enemy.
To this day, I refuse to use fire pigs, and any AI faction seen using them is presented by an act of war.
FUCK.
FIRE.
PIGS.