Fire one off the rail as soon as possible, make him go defensive first, if he doesn't budge, fire another one, by now you're probably right outside or maybe inside the NEZ for BVR where the missiles will have a higher probability to kill, well you better have your chaff and flare keys ready to go, because they probably evaded your missiles at this point, and he's probably turning back onto your nose, you might have a higher chance of shooting another missile off when he's trying to turn in, forcing him to stay defensive, and if he's smart he'll break off entirely and try re-engaging at another angle. OR he could have fired at the same time you fired your first initial missile, so both of you would have to go defensive, and things can get a bit tricky, and i'm still trying to figure that part out myself.