I seriously doubt the killer elephant is going to go all benign on us just from removing the weight from its tusks or idly chatting with it. It's out for blood, and I think we'd need something better than pleasant conversation to calm it. Probably something better than good herbs as well.
For the moment, we might or might not be able to assume the rest of the herd is right behind the murderous one. We probably can assume that the herd has been following us under the direction of their new chieftain. As I see it, we really only have one option, depending on how well I understand tactics and rivers.
If it's just the one elephant, we can probably assume he'll value elf carnage above just about anything else. He was rather cautious in waiting for the proper time to attack, so he's not just a berserker, but between rescuing his caged fellow and murdering us all in an unholy rage I'd say he'll take the latter. Which means the best course of action, I think, maybe, would be to have the sentry hurl the torch in his eyes to buy him a few moments, then have every elf dash across the river, leaving our goods behind.
The bowelves will of course begin firing from across the river as soon as they've arrived- they'll probably be the fastest across anyway, and with luck this will also help convince the elephant to cross the river and murder us over there as opposed to hunting down the straggling civilians. We position our spearelves in front of our bowelves right at the river's edge, however, so when it thunders across, taking arrows the whole way, it finds itself trying to fight while in the river. That should give us enough edge to either drive it off or kill it.
Problems with this plan include that it assumes we're very, very close to the river, that there's only one elephant or that we can intimidate an entire herd away, and that it will die or flee rather than smash all our goods on the opposite side of the river, though if we're close enough to get across the river in the first place we're probably within bowshot from the other side. I suppose we might also have trouble seeing across the river into the darkness, but Amala and her rangers are probably accustomed to doing something about that.
Mind you, I would like to enact a diplomatic solution, but you've seen those eyes. I just don't think our animal taming skill is up to that level.