Assaulting expendables with traps and beasts. Mangling the unwary, quelling the DM blood-thirst which rises....
If you maul and murder too many of the Cavemen you can have mercy (let your players sweat it, don't tell them you actually have mercy and a backup plan) and boost some numbers before the final crawl into the fire caves.
The Fire Apes have ranged far and wide ravaging othe tribes as well. If they need a few extras to boost the numbers its entirely OK to have a few members of one of these tribes encounter the player Cavemen and join up to head into the finale.
It's also pretty railroady but this kind of thing is really just an excuse to sit back, throw dice, and try not to die last. Character development is not a high priority. It's not meant for long-term character driven play. It's a lot like a horror or survival movie. Its meant for one or two shot amusement and a big part of that amusement is trying to stay alive to the last scene.
This is just a bare-bones frame: PLEASE liven it up a bit or your own players. Random encounters (God knows there's enough charts out there), travelers, natural disaster, Gonzo, whatever trips your player's trigger. Go for it.
So let's get to it:
First a crappy homemade map!
The Cavemen's home cave is in the north hills. The Fire Apes raid from the Fire Lands, beyond the southern edge of the map.
The Cavemen live in a nice hilly region with plenty of game and vegetation to forage. There are a number of caves to protect them from the weather and life is good.
The Fire Ape raid caught them totally by surprise. The apemen had never journeyed this far north before.
Their tracks lead south, towards the river.
I'm going with a series of set encounters for this. Add as needed.
An important consideration for the party will be rest, food, etc. A night in a wet swamp surrounded by snakes and eaten alive by mosquitoes is not good rest. Penalize actions and no healing til they do get a proper camp.
A Caveman's life away from the cave is tough. At least game is plentiful until they get to the Fire Lands.
An hour's travel from the cave.
Nearly a hundred feet wide. It is heavy with fish and almost 30' deep at the center.
Crocodiles and snakes occasionally hunt this bend of the river.
If the Cavemen choose to swim, as opposed to say floating on a log, they will have to make a Strength check to avoid being pulled under by the current. IF they are pulled under they must make Saving Throw to reach the surface or begin drowning as per your rules of choice.
If you need to throw in some excitement throw a crocodile at them as they reach the other side of the river.
If they've annoyed you throw three or four at them.
The forest is a half days journey wide.
It is lush and thick with trees, bushes, and furry animals of all sorts.
Perfect or a roaming pack of remnant Raptor dinosaurs out to get some Cave meat.
Sweet sweet Cave meat.
One per player should be plenty to rip up a few Cavemen and scare the crap outta the rest.
You've seen Jurassic Park, you know what they look like.
OH, btw, This is their home territory and they are fantastic hunters: they gain surprise on a 1-3 on a 1d6.
AC: 5 
Attack: +3 to hit, two claws ( 1d6) or one bite (1d8)
Saving Throw: 15
Dead Raptors have fantastic hide for armor (+3), and their claws make excellent daggers (1d4). Plus they taste great.
The swamp will be another half day's journey, possibly longer with the hazards and treacherous footing.
Miles of crocodile and snake infested stagnant water, trees, and the occasional island poking from the muck. An ancient pillar or pile of cut stones pokes from the waters or lies scattered on the islands in various areas in the heart of the swamp.
A dying race of amphibious frog-men. They lair in the stone rubble on the islands at the center of the swamp in dank, pools lying beneath the toppled walls and columns. They have no treasure, only bones and other natural trinkets, and the females stay here, protecting the few viable eggs which are laid in the pool. They do not venture outside of the swamp and were heavily decimated by the ravaging Fire Apes, though they managed to kill a few.
When the Cavemen wade through the muck approaching the domain of the Swampies they will see three dead apemen strung up on crude crossed poles.
In addition to the hazards of the swamp the Swampies have also heavily trapped the area leading to their domain.
Every turn the Cavemen must roll a 1d6. On a 1 or 2 they have encountered a trap or hazard. The trap or hazard can be avoided with a successful Saving Throw.
If they fail the Save the trap goes off or they blunder into the hazard.
If it is a trap or if they make a lot of noise blundering into the hazard they alert the Swampies who will arrive in 2d6 rounds.
Possible traps and hazards:
⦁ Snare: the Caveman's foot is caught in a vine snare, jerking him into the air 20' where he takes 1d3 damage and dangles helplessly until he saws through the rope (a Dexterity check with a -2 penalty) or a companion climbs up the tree and cuts him down (at 0 penalty.) The time to cut down is 1d4 rounds (add 2 to this if the Caveman is cutting himself down.)
⦁ Spear trap: a massive log spear flies out of the dense swamp and hits a Caveman for 2d6 damage. It is large and awkward and his movement will be 1/2 until someone makes a Strength check and pulls it ot of him.
⦁ Spiked log: a log swings out of the trees and slams into the Caveman, doing 2d8 damage and sending him flying 20' back down the trail, possibly causing his companions to make Saving Throws to avoid being hit by their flying kin. Anyone hit by the Caveprojectile takes 1d4 damage.
⦁ Quicksand: Save or get sucked down. Your friends have 1d6 rounds to make three successful Dexterity checks at -4 each (one Caveman or several cooperating, as long as they hit three successful checks) get you out or you die! If any of the rescuers rolls a natural 20 they fall in and suffer the consequences.
⦁ Poisonous snake: attacks the Caveman. +1 to hit, AC 10, 3 hit points. If hit, Save or die horribly in 2d6 rounds!
⦁ Green Slime: A pool of floating green slime on the water. It attacks any Caveman who blunders into it. Good luck Cavedude!
Bipedal naked frog guys. A sad dying race who are pissed off at the world. Throw quite a few at the Caveman. It'll make them feel good to kick someone else's ass for a bit. Especially if they took some hits from the traps and swamp hazards.
AC: 8 
Attack: +1 to hit, spears (1d6)
Saving Throw: 17
Move: 90/30 on land, 120/40 in water
The Swampies have an irrational fear of fire. If even one torch is lit the Swampies will have a -2 to attack.
The Giant Snake and the Stirges
Two natural predators in the swamp. They stay away from the Swampies island for the most part as they have learned the Swampies spears and traps hurt!
When you need something to spice up the final bit through the swamp, throw one or both at them.
The giant snake would be a fantastic night enconter. They have to eat and sleep sometime.
Eat a couple.
The stirges would make good 'race to the edge of the swamp' encounter. The very last kick in the furry pants. Wearing them down for the finale to come. Fire is your friend.
Statted in almost every edition and clone. Look them up.
The Hills of the Ants and the Cave of the Fire Apes!