I, in my opinion, think that trying to tame the elephants we have into war elephants is an absolutely horrible idea, and here are four good reasons for that belief:
1. I do not think that we can tame the beasts, whether by threatening them by death if they do not fight, commanding them to fight, or by promising them treats if they fight. It just will not work, and even if it does, it would take way to long for our purposes, which brings me to the second point:
2. It would far take to long to tame the elephants into war elephants, and I highly doubt that the frog men that are already attacking us are going to wait around as we try to tame these beasts. By the time the elephants are theoretically "tamed", the attack will be long over.
3. I do not believe that if we do release these beasts that they will charge at the enemy. Rather, I feel it is more likely that the beasts will make a run for it to the plains behind us, and thus by doing so they will run along the narrow piece of land that we came in on, where there are few or no frog men, but many elves and mules.
4. Even if we somehow manage to tame the elephants into beasts of war and mange to get them to charge at the frog men, I feel that there is a minuscule chance at the beasts being effective weapons against our enemy the frog men. The frog men would either let the elephants get stuck in the mud(thus allowing the frog men to freely kill the beasts), quickly use their spears as javelins and kill the beasts before they do much damage(okay, this one is less likely to happen, but it just might), or they will just run away and leave us with the awkward position of trying to get the beasts to go back into the cages that they have been put in. Remember that we do not want to lose the elephants, because then we will not be able to trade them with the dwarves.
I think we just simply fight them off. After all, we are physically stronger(I believe we are at least) than them and we have the better weapons. As long as we keep formation we should win. Also, we should use our bows against them when we can, but make sure that we will not accidentally hit other elves in doing so.