Yes I have. I have only hunted Loksodonto Africana African elephant.
First thing is to remain calm, panic and run you are dead...(sometimes you have to run depending on the situation and the terrain but experience will guide you).
Gain experience which takes a long time, study elephant behaviour and signs, they will let you know when things are not going to their liking, understand these and take appropriate action.
When the pawpaw hits the fan use enough gun, with the right bullet, shoot straight(you have to know exactly where to aim to hit the brain), most people mis high....
Un wounded elephant can be stopped or turned by, raising your rifle above your head, shouting, shooting over them, putting one through the ear or even high through the head over the brain. However again experience will determine the right course of action in any given scenario...
My choice in bolt action is 500 Jeff never let me down, in a double I would use a 500 NE, that is as a guide and PH not a visiting client.