The name of this achievement is "Elevator Action". It is a MISSABLE achievement; if you make it to the top of the large elevator sequence in Chapter 7 where the Tripods are attacking you through the windows and you don't get it, you will need to reload an earlier save. If all of your save files are past this point, you will need to do it on a different playthrough.
To get this, you need to knock all of the Tripods off of the large elevator that you ride in Chapter 7. By my count, there were 18 enemies that attacked. I got this on my first playthrough on Normal using the non-upgraded Pulse Rifle's Alt-Fire. Just make sure to buy enough ammo so that you won't run out. Each alt-fire costs 25 ammo, and you'll need a minimum of 18 shots. I made sure I had 20 shots just in case. For those of you who don't want to crunch the numbers, 18 shots comes out to 450 rounds, and 20 shots comes out to 500. Once you have enough ammo, just make sure you shoot the Tripods quickly and always reload after shooting one, unless there is another enemy right next to him. You can only fire twice before you must reload, so it's better to reload when it might be unnecessary rather than get caught having to reload when two enemies show up at the same time. It only takes one shot to knock each one off. Sometimes it looks like they're still hanging on, but trust me, if you hit them, they will fall off. Again, I did this on Normal, and my Pulse Rifle was not upgraded at all. Not sure about higher difficulties.
If you dont have quite enough funds to get enough Pulse ammo, you can also knock the Tripods off with one shot from the Detonator. Between these two weapons and maybe selling some of your extra items, hopefully you can get enough ammo to last through all 18 enemies.
This acheivement really wasn't as difficult as I thought it would be. Good luck!