Specialty unit, I think. Much like you keep cavalry away from the front of a phalanx, keep elephants way, way away from skirmishers. I'm pretty happy though with what elephants can do when they crash through most units, especially low-morale units - it's impressive. Though, like cavalry, they are a little more vulnerable when they lose momentum inside of a heavy infantry unit with a strong morale, I guess. I'm trying to remember, weren't elephants eventually considered almost obsolete in history, once the Romans or whoever figured out how to fight them?