It's Art I Tell You
The fact that Karl was aiming to shatter the Giant's weapon right from the first strike seemed to make it obvious that he had a weapon shattering technique, but [Shatter] didn't only work on weapons, but on anything that it hit.
The vibrations would cause bones to break just as effectively as it caused a wooden club to break.
That distraction let the other warrior get behind the target, and score the first strike of the battle, cutting a line down the Hill Giant's lightly armoured back as Karl kept it distracted.
The Giant turned to swing at the agile warrior, and Karl backed up, switching back to his bow to plug a [Flaming Body] infused arrow into the enormous target.
It was like shooting at a house from the middle of the lawn, impossible to miss.