.. How do I avoid the boulder?

After you enter Tartarus, go forward till you see a door with an evil face and a ramp going up to the left. Walk up the ramp --

-- and yikes, a boulder! But it's easy to avoid getting squished if you're quick.

As soon as you see the boulder start to wobble, back up hurriedly, and Nancy will leap out of the way.

That's the only time you have to worry about that boulder. This isn't Skyrim; the traps don't automatically reset, thank goodness.