After critizising the battle tactics, which I still think was over the top bad, I've been considering the possibility that they've made them bad on purpose.
A running theme throughout the show has been to kill off everyone remotely capable of leading a battle, espescially the heads of the biggest house from each part of the world; Oberyn and the prince from Dorne, all the Baratheons, Ned and Robb Stark + their advisors and the Karstarks, Umbers, Boltons the Freys and the Tullys, the Tyrrells, the Tarleys, Tywin and his brother (and they earlier destroyed the second largest house in the westerlands). Also lord commander Mormont and the king of the Iron Islands.
Of those left who could lead armies were Jamie, who was critized by his father for spending most of his life as a glorified bodyguard and maybe the commander from the Vales, although they lost their leader, Arryn, and didn't really take part in any battles.