Metal Slug XX

inspired truth's solution is spot on. I have a video which executes how to unlock this achievement perfectly! Please enjoy!

Of course, you don't have to kill all the soldiers if you don't want to as the achievement unlocks as soon as they appear.

for this achievement you must destroy 3 boxes in level 2

in level 2 go down the first elevator
first you must destroy the box to far right with melee cobat
then a second one apears on the top left destroy this one and then the lower mid one

then the 100 MORDEN Soldiers should appear and the
achievement pops up

To help with your description when you get on the floor with the crates do not kill anyone. Just head to the crate on the right and destroy it with melee, then go to the one that appears where the guy with the siren is (you can kill him when you get the crate) and then get the bottom on. (Sometimes you dont have to get the bottom one for the effect to appear)
Posted by Shi Kiseichu

The 2 other solutions here did not work for me. After trying for this for 1/2 an hour, I think I figured it out.

After going down the 2nd elevator, DO NOT kill any enemies. Make your way to the crate on the far right, and jump above it and shoot it to destroy it without killing the enemies around it.

Then make your way over to the 'new' crate on the platform next to the guy with the siren. Destroy the crate and the enemy with a melee attack.

At this point, a power-up appeared; and the screen flashed the "100 MORDEN Soldiers". And there are seriously a grip of enemies to come....

Hope this helps others frustration with this.

--enjoy

This is NOT for rescuing all the POWs.

In level 2, you will take some elevators to go through the level. After the second one, stop killing soldiers and run to the right where you will see a crate. Get close to it and knife it. I think it drops a kitty, which I picked up. Then go to the crate to the far left (I think it drops down after you knife the first crate) Knife this crate as well. Then go for the crate in the middle and knife that one. Now start killing everyone. At some point, the achievement will unlock and soldiers will start falling from the sky.

I'm not entirely sure if you can't kill any soldiers while you are knifing crates nor am I sure if you have to pick up the "treasures" that were in the crates or if the crate order is important. I did it just as I described above and it worked, which was good enough for me. I'm pretty sure I died between crates so I don't think that matters.

Just watch the video. I found it on youtube.
It is very easy, I got it in 1 minute thanks to it
It is really quick !