I'm thinking our plan to trap and then rescue the elephants was a bit hasty. Our response to them catching us in the act was definitely hasty.
So what have we learned? Among other things, that elephants are fairly smart and perceptive. So, let's use that against them.
The way I see it, there are now two potential types of elephants: Gentle, benign elephants and bloodthirsty marauders. Presumably most elephants are at least vaguely the first, while I would imagine the one that got our two elves is leaning heavily towards the second. Depending on which we run into, our tactics should differ considerably.
So here's what we do: We set up something interesting out in the open. When the elephants notice it, they come over to investigate. If "investigate" contains the phrase "gore", the elves responsible rush back to the trees- but while they're setting up the interesting thing, other elves dig a pit and cover it, so the field elves have an obstacle to put between themselves and certain death. Furthermore, since digging a hole in the ground near trees is less noticeable and interesting than whatever we'll be doing out in the open, the elephants shouldn't even notice the trap.
So, what's this interesting thing we'll be doing out in the open, and how does it catch elephants? I, uh... I'm not sure, actually. Maybe it could be a very, very sturdy cage with widely-spaced bars, so if an elephant charges an elf in the cage, the elf steps backwards at the last second and the impact jiggles the door free, snapping it in place. If there's multiple elephants or they're too wary to enter an obvious cage, the pit could cover our valiant elves' escape. Ideally we'll also be able to trap the now-pitted elephants, but if there's not enough time to get the pit large enough we should at least be able to avoid casualties.