Forge suggests we position ourselves so that we can strike the snake as soon as it appears in response to the mage hand. Forge does so but of course doesn't touch the water. Readies brash strike for when the trap appears.
Ghen runs out to the river to help Buckthorn. While running, he pulls the rope from his backpack. When he gets there, he'll throw the rope to Buckthorn so he won't wash over the falls.
Takes up station on the opposite side of the pool from the dwarf. Draws his blade with a sly flourish (pun intended) and in a taunting whisper to the axe wielder says "First round at the next tavern says I slice it open before you."
The planar flails his sword around in frustration as he bickers at Nar's swiftly receding back "We're tryin' to arrange a soddin' ambush here ye' pikin' berks!"
Buckthorn does in fact fail to swim, or catch the rope (and throwing and catching under these circumstances probably pretty hard, but you can try again). He moves 20 feet closer to the falls, which are just over 20 feet away.