Like Utopia already said, the elephants doesn't really sound much when walking. Near well nothing at all actually, at least the ones I've seen at zoos here in Sweden. They have a very effective gelatinous cushion-sole under their feet with some pretty amazing shock absorbing properties! I actually got MUCH more interesting movements from cows...especially one humongous pointy psychopath who came to the conclusion my ass was grass & she could do with a snack... Jeez...
If you still wanna make a more Hollywood-esqu sound for their feet I'd say foley (much faster) or some creatively designed sound editing (more control) with excessive use of pitchshifting and bass-processing/LoAir is pretty much the only way, but as most people knows pretty well real elephants doesn't really make the ground tremble, I would probably solve this by gaffa-tapeing skin-cushions from sofas to my feet, or perhaps regular cushions wrapped with leather jackets and then duct-taped to my feet, recording at the nearest (undisturbed) crappily sustained lawn or earth-patch. Or, of course, in the studio if you have the possibility of real foley :-)