Whether a one hand grab or two hand grab from front or rear, you should immediatly trap the attackers hand on top of your weapon as tightly as you can all the while twisting from side to side with your hips as fast and hard as possible. First and foremost before you can attack him you have to keep him from accessing your weapon. Get free and get distance from the attacker. Once you are free you can make the decision to use the necessary level of force. You can practice this with a dummy weapon and a friend. Make it real, be forceful. It will give you a reality check especially if you are one of those that uses an open top holster. Once again, that is why it is so important to be aware of your surroundings, and better to be concealed, if you have an easy access holster.