I played a few custom games with elephants to see what they're like.

I found that the computer tended to focus fire one unit with all the missile units it had (switching archers to flaming arrows) this was enough to make one of my ele's go amok within about 30 seconds.

I also found that there is no infantry formation which can stand up to ele's. Pikeman, axeman, voulgers whatever the elephants kinda charge through (works best with a bit of room) and don't even slow down no matter how dense they other group is. So i doubt bridge battles are going to help much... though the time they spend crossing the bridge might help.

I only played with the ele's but the idea of javelin cavalry sounds pretty good.