I have never experienced a charge from an elephant or buffalo, but I have shot several buffalo using frontal shots, both with my .505 and with my .458. In every occasion the buffalo succumbed to the shot, but not immediately. My only experience with a charge was with a black rhino, and true to what I had read and heard, turning him was not a problem. In fact, he did as neat a 180 degree turn after the first shot as I could imagine.

My advice would be to use as heavy a caliber as you can comfortably shoot with under stressful conditions, keeping in mind that you will be pumped full of adrenaline when the time comes. Accuracy is much more important than brute force. Blazing any number of shots into an animal's carcass is far less effectual than one shot in the right place.