"Hm? There seems to be some rearrangement in Amako's army."
"My lord, Amako's first line is now spear units. Their bow units have backed off to the second line, protected by spear units on the flanks. There is no more third line."
"Oh. This is the famous Porcupine formation. Anything that has melee units up front is a blessing for Bait Tactics. "


Note: Amako starts the battle with 15 spear units and 4 bow units. The formation shift happens when there are about 8 untis of spear and 4 units of bow. Apparently, AI chooses its formation based on their current army composition.