I don't even bother with elephants to use as a direct killing unit. I use them to break up any lines which are a bit to thick or have units which I don't like. Otherwise they sit on the sidelines to knock holes in walls and stomp the occassional unit that tries to flank me. After having a mercenary elephant unit kill my 8 star faction heir I decided that it wasn't wise to use them as a sustained shock element. :) Not very fun paying upkeep on a unit to have it kill a general. Mind you I should have waited a bit longer to have charged my heavy cav in for the kill, but I was playing with an imposed 10 unit per army limit (to help the AI since it doesn't have that limit), so my cohorts were quite busy elsewhere at the time.