Atlas essentially has two passives – one that has a massive impact on his survivability, and the other that’s more of a Quality of Life passive. I’ll let you work out which is which.
This passive means Atlas can’t be knocked down while he is in contact with the ground – although he can be knocked back by certain enemy abilities. He can be knocked down while jumping, though, so maybe best to keep his feet on the ground when Eximus enemies are around.
I’ve mentioned Rubble a few times already, so it’s time I explained it, I guess. All petrified enemies, regardless of the method of petrification, will drop Rubble when killed, as will Rumblers on explosion. When Atlas picks this Rubble up, it will restore 50 Health or 50 Armor (only provides Armor if at full health). This stacks to a maximum of 1500 Armor, but note it decays over time.
Petrified enemies killed by Landslide will grant an additional 25 Health or Armor, so it’s also important to keep that in mind.
As Atlas gains additional Armor, you will see a noticeable build-up of rock on his body, along with a circular gauge on the right-hand side of the screen. You can use this to monitor the decay of Armor. (Tip: Rubble will be tinted with your warframe’s energy colour, so if you are having trouble noticing it, try making your energy a brighter or more vibrant colour.)