Well, elephants themselves do not cause that much damage even if they charge head-on. They trample units and trampled units can stand up afterwards and continue fighting. The real damage of the elephant is destroyed formation which can be exploited by troops charging behind the beasts. At least, that's how it is in the game. As to being overpowered: I do not think so. I could deal with elephants (separating them from the army and then driving around or even routing them) before the patch. It's the AI, that has real problems dealing with human led elephants...