Beware the silent jungle, for it knows what you do not.
The Undershadow goblinoid tribe are well-adapted to threats from above and below. The skies are ruled by "terrorhawks", massive aerial predators that can see as clearly at four miles as they do at four feet and can hear the grunt of an ox through a hurricane. The ground floor is ruled by a wide variety of voracious predators that would like nothing more than an easy goblin snack.
So in tune with their environment are they that they have learned to make use of their predators in defending their home. Woe betide any adventurer who thinks goblins are easy pickings in the Whispering Wood.
Undershadow goblins, even more than normal goblins, rely on stealth and cooperation to survive. They are trained from birth not to make any sound that does not mimic the sound of rustling tree branches or creaking wood, and have made an effective language out of hand or foot signals and pushing or pulling on the ropes and planks that connect their homes.
What is more, Undershadow goblins learn from an early age how to navigate the their village either upside down or right-side up, the better to avoid a predator that might spot them from above or below. They are just as adept scampering underneath a bridge as they are running over it. These goblins grow out their nails and harden their hands into sturdy climbing claws, which also function as handy backup weapons in case they are ever cornered.
The last major adaptation the Undershadow goblinoids have made to the constant perils of their environment is the development of highly effective full-body camouflage. Not only does this allow them to blend in unerringly with their home lands, this also functions as one of the few methods of self-expression in a society where normal arts and music are the easiest way to get eaten. To the untrained eye, a group of Undershadow goblins might appear to be an unassuming section of burls or knots on a tree, perhaps a mossy growth; a skilled observer would see marks of clan, family ties, bonds of friendship, social status, and individualism.
When defending their home against invaders and adventurers, Undershadow goblins eschew direct combat. A few of the larger members of a tribe are designated distractions, tasked with staying alive and keeping a foe's attention while the rest of the tribe scuttle underneath bridges or across vines to set up a new trap.
Undershadow goblins take advantage of their natural predators in two primary ways. The larger distraction goblins gang up and pin the adversaries that seem more nimble and sure-footed on the swaying rope bridges. They are assisted by a sticky resin that is smeared on their skin at the first sign of trouble, as well as blowgunners firing darts tipped with a soporific poison. Once an intruder is bound properly, they are hoisted high into the trees, above the canopy, and the goblins begin hooting and hollering to ensure the attention of the terrorhawks.
For creatures that seem to favor the stable footing of the tree trunks and larger boughs, the solution is much more straightforward: trapdoors and goblins scurrying underneath the branches wielding nasty hooks are quick to deposit their visitors on the forest floor. If the tremendous fall doesn't kill them, the hungry predators will.
While the village is their home, the Undershadow goblins are just as at home in the unimproved trees that surround them. If things are taking a turn for the worse, they dump their valuables down to the forest floor and flee through the branches. They would prefer to risk a few quick raids against known predators than be wiped out by strange invaders.
This encounter can be scaled to accommodate adventuring parties in the low- to mid-range of levels, when goblinoid creatures and great falls are still threats. The saving throw DCs for the poison does not need to be very high, especially if it has to be made several times a round. If you want to really throw the players for a spin, consider including a few shamans or spellcasters that can control the vines around the village, in the style of an animated rope, an assassin vine, or a kyton's chains.
The goal of the Undershadow goblins should never be to kill intruders in straight combat. They prefer to let the much bigger and more deadly denizens of the forest do that part.
Higher-level adventuring parties will find dealing with the Undershadow goblins rather trivial, at least without resorting to using an amount of goblins that is more appropriate to a wargame. If you still want to challenge a higher-level party with the Undershadow goblins, consider starting the adventurers already on the forest floor (in combat with a much larger predator or five) or already in the air (in battle against a roc), and play up the effectiveness of the poison darts and the camouflage. Nigh-invisible enemies that get sneak attack and poison dice, and which can move through the trees silently faster than you can run across the ground, is an excellent tactical wrinkle to eat up a high-level party's resources.