Any you are within your legal rights to do so, as long as the threat is reasonable.
How about pounding on my window with a helmet? Slashing at me with a knife?
Even with them just pounding and damaging the vehicle I could easily say I was in fear of my life. Anyone close to my vehicle trying to break into it would be a valid target. A biker sitting back 20 yds watching would not be a valid target.
Lets say someone hits the back of my car causing minor damage. I do not have a right to attack that person.