Mountain Base - Non-Believer Banner #4

Go through the doorway on the far wall, opposite the entrance and below the graffiti that reads, "Embrace the Flames." In this little control room, you'll find a salvage crate, pistol ammo and another banner (4/5) for the Non-Believer challenge.

You could go all the way back to the bathroom for some fire, but that's not necessary. There's a closer source. Exit this room, turn right and go up the stairs.

There's a flaming barrel at the top, but before lighting your torch, ready a weapon.

When you take a few steps into the hallway, another Solarii drops through the grate on the ceiling to your right. Shoot him.

Then light the torch at the barrel...

...and return downstairs and left...

into the little control room with the challenge banner.

Set it on fire. Then pick up the goodies if you didn't do so before.