I just checked, the archers, lions and the formation are all one squad (enemy group). The formation is supposed to walk back and forth, along the stone wall and will retract its spears when it reaches the side to the rocks, but will only do so if all other enemies are at their position. It's difficult to explain, but sometimes it can become stuck and not move back to the other position, infinitely keeping its spears out. If you kill the archers + lions it should work fine, though?
