Be Quick or Be Dead
This is a one-time challenge. During the Let's Build a Robot Army story mission, you will come to a point where you see four Scavs standing outside of big door. When you get close enough, they will run inside and get into Powersuits with which to attack you. You will be required to kill all four of those Scavs before they manage to get inside, so it's best to use something like a rocket launcher to kill them.
You will need to go through the entire zone without using a single Air Dome Generator to replenish your oxygen. If you don't know, these are the structures you use by interacting with them, which will create a dome of oxygen. That means you will need to rely on the little oxygen canisters that are loot, as well as the indoor areas for oxygen.
However, there is a super easy way to get this challenge once you've gone to the Titan Robot Production Plant area. As soon as you enter it, use the exit to head back to this area and the challenge will pop up as complete.
This is a one-time challenge . During the Let's Build a Robot Army story mission, you will enter a Stingray garage, so make sure you have something like a rocket launcher ready. On the opposite side of where you entered, there will be a door and some Lil' Scavs will be there. You have to kill them before they manage to close the gate, so you only have a few seconds before they do.