Elephants? Ooh okay.

1) Fire arrows: I don't know why you say they don't work, but they do work, they do work rather effectively.

2) Javelins: Just get some Cavalry Auxilia, and they'd do the job.

3) Morale: Eles morale attack is too strong, yet their own morale is too weak. One simple thing to do is to surround them with your units, usually getting them stopped by 1 of your infantry units, then swarming them from the back with cav will rout them.

4) As you already said, just get an onger flamed rock to land on one of them :)

General Tips:

When fighting an ele, it's best to have your men set to loose formation, minimizing the losses you can get out of this fight. Eles are quite not effective alone, if you can lure them out of an army then swarm them, your troops will make a short work out of them.