"You're weak, Kal-EL. Unsure of yourself...."
Clark's whole life, he was told to do nothing but hone his sense's and blend in to a society that he has severely passed by. He wasn't even allowed to defend himself and stand up to bullies and when he did show himself, it was saving others from death and/or destruction, not even any kinds of combat whatsoever.
Over night, literally, he's all of a sudden thrust into a world-changing event that immediately needs his assistance in not only war and combat, but combat against his own kind, who happen to come from a world that is not only highly advanced, but advanced as pre-conceived military warriors who were bred to fight and...ultimately....kill.
I'd say inexperienced is an understatement.