Harold should hardly call himself an amateur. He has been training with Itsuki, who has got the best swordsmanship education the Sumeragi domain can offer. Harold has also been fighting, along with the soldiers, against monsters for multiple years now. How exactly is he still an amateur with nothing but the original Harold's specs and his own game knowledge? He's a battle-scarred veteran already.
It's pretty obvious the knights' training is lacking, however. If Harold really wanted to train those three, he should take them out there into the wilds, with no larger army to back them up, and have them face monsters and hardship, have them make decisions that could decide between their life and death. It seems like even Cody isn't especially experienced, in fact, or maybe he's just a total snake and acts like a noob to trick answers out of Harold.