Far Cry Classic

This has to be done in the 19th level called "Dam". I'd highly suggest that you do this on the easiest difficulty. I'm not saying it can't be done on a harder one but why make life harder for yourself? It also doesn't matter if you die and have to reload a checkpoint!

Quick note: Clad master noted in the comments that the achievement didn't unlock while playing on hard. But after he switched to Easy it worked just fine. So if you're having problem unlocking it, try switching the difficulty and try again!

You start this level without any weapons in your inventory and you can't pick up any. It's not as hard as it sounds though.

When you start the level run straight ahead and dodge the single trigen there. Jump down the waterfall and swimm along the river. Don't sprint. You want a full stamina bar for the next part.

Follow the river and don't pay attention to any of the enemies around you. You want to haul ass here. So sprint because your life depens on it! At the end of the river turn right to trigger a cutscene that will take you to a downer helicopter.

Grab the armor to your left and use the mounted machine gun in front of you. Mow down every enemy that appears on the beach in front of you. After you've killed all of them turn around and grab the other mounted gun to clean up behind you.

After you killed everything you can progress up the hill. Stick to the right side of the hill and sprint all the way up. Get into the fortress and turn to your left to enter an armory.

This will trigger another cutscene. After you've watched it grab some more health, armor and the key card. Make sure to NOT grab any weapons in there. Head outside again, open the gate and jump into the jeep there. You can kill enemies with the weapon but you can also driver over them.

Follow the path with your jeep until you get to a lava stream. On the upper end of the lava you can cross it by using a fallen tree. Get over there. You'll be facing another stream of lava. Again, up the hill to cross it over a couple of rocks. In front of you there will be fight. Just run straight and you hopefully won't lose much health.

Now get down the mountain and you'll only have to get up one more hill and the level is done. Stick to the right side of the hill. There will be a hole in the fence where you can slip through. Sprint up the hill to reach the building at the top. Follow the stairs and get to the massive door with a bunch of soldiers in front of it. Book it to the door to end the level and get your achievement!

I did this on Hard.

Run down river, evade the Trigen, and then leap off the waterfall. Continue to follow the river while evading more Trigens. A cinematic will be triggered when close to the destroyed helicopter. After the cinematic ends, grab the mounted gun and kill the two Locusts (the Trigens that jump around). Ignore the Fat Boys (the big, lumbering Trigens), because there isn't enough time to kill all of them on Hard.

Exit the rear of the helicopter to avoid the oncoming Fat Boys, and then sprint around it toward the fortress while evading more Trigens along the way. Enter the armoury and pick up the keycard; use the keycard to open the gate and jump in the vehicle.

Drive along the broken bridge and down onto the grass. After running down the first Fat Boy, there should be a slight rise to the right that can used to jump across the river of lava. The grass leading to the jump is a lighter shade of green and it's well before the fallen tree. Jump the lava, rejoin the road and just drive, ignoring enemies along the way.

Near to the objective, the road dips and then turns right, opposite a circle of sandbags. The mercenaries have set up a roadblock, but it's possible to squeeze past between the concrete wall and a tree. Slip past the roadblock and continue along the road as it bears left, running down mercenaries along the way. Exit the vehicle near the base of the steps, and then sprint up to the large door while ignoring enemies.

There is a mercenary with a rocket launcher near the base of the steps, so try and run him down with the vehicle before exiting. Moreover, when ascending the steps, go right rather than left, because it's the path of least resistance. The mission ends when Carver reaches the door, so don't bother trying and kill the nearby mercenaries--just get to the door! With a bit of luck, you'll get there in one piece.

Two others solution are great but i want to give you more tips.

I did it on NORMAL, and on the first attempt achievement didn't pop due to... throw a rock ! So to be sure you do it right just avoid puch .

Second time i focus on methodical progress and,
i don't want to copy great work other solution authors so i pass to point when you enter to Buggy... do not enter the Buggy - to be precise
Just walk down without any unnecessary speed and head to fallen tree. Across lava and next across rocks.
Then run between shooting guys and climb on hill with radar. Be careful a do not grab Sniper Rifle. Pass through hill and head slightly right, go down, to road.
Jeep with three guys should arrive and crew should leave car. Quick steal vehicle and kiil all you want.

Now the last part.
Ride to the roadblock and kill all enemy (rockets are pretty ). Go to Armory (building on the left and grab armor). Back to Jeep and fire rocket at the final building. Enemy should leave fences and try to kill you - how sad, cause (on NORMAL or EASY) they really easy to kill. Now take a walk to final door. Maybe some enemy still waiting for you(in cover), but with full armor and full sprint bar you may take beer a finish this level with one hand.
Achievement pop !
And if you die, just reload last checkpoint all still will works.

I hope that was hopeful, especially if someone don't like insane runs - like me.

Feel free to comment.

Happy hunting !

After dozens of attempts finishing the game without killing the enemies near the door, and sometimes dying to them and ending the game. I used the car to kill everyone around and reached the door with everyone there dead. Achievement unlocked.