I'm pretty sure Bob the Insane is right on about the hit points being most likely shared by mount and rider, and that's what makes them nigh invincible. It's very apparent when you watch were the majority of your arrows land. Missile units fire at the elephant itself, and even when a rider is struck, the hit seems to come off of the total. Has anybody ever seen an elephant with dead riders on it? Maybe I'm wrong here. There have been times when I've wanted to be able to train a Legolas unit to take care of the elephants (though I don't remember him being so acrobatic in the books).