What levels of force may a person use in defense of others?

Only as much force as is necessary to stop the assailant. In defending yourself or another you cannot use excessive force against an attacker. For example, you cannot take an attacker's life unless it is clear he will, by his actions, take yours or another's And you must be able to prove in a court of law that such force as was used was necessary, otherwise you can end up in prison for a long time. Also, after you have stopped the attacker with only as much force as necessary, you do not wait around and wait for him to get back up and start where he left off. As soon as he has been temporarily incapacitated it is your legal duty to try to find an avenue of escape. It would also help if there are witnesses who can testify that you had to take whatever action you took, and unless you know the person being attacked you may actually end up aiding the one who actually started the fight.