Hitting the glass with a club or bat IS attempting an unlawful and forceful entry and I would be well within my rights to use deadly force to stop their attempt. At what point would I draw? At what point would I fire? If they were standing on or beside my trapped vehicle with a raised club threatening to strike, they would be staring at the business end of my gun with full knowledge of what will happen to them should they proceed. The moment that the glass cracks, breaks, or otherwise makes a clear shot, is when I would fire. I would hope to have the presence of mind to take a picture of them making the threat, but even if i don't the physical evidence would speak for itself.
The person against whom the defensive force was used was in the process of unlawfully and forcefully entering, or had unlawfully and forcibly entered, a home, motor vehicle, or workplace, or if that person had removed or was attempting to remove another against that person's will from the home, motor vehicle, or workplace.