To get to Short Stack (Charlier from the XBLA game Stacking), you need to drop off a cliff. This cliff is found south of the Trowbog Elder. Go over the first ramp, then the second but before the third (Everrett will comment on a satellite view of repugia), there is a cactus style plant on the right, you simply need to walk between the two plants and off the cliff and then enter the cave at the bottom of the drop, the achievement will pop after that.
The Elder in question is the second Elder, the TROWBOG, not the GRUBBIN Elder. He asks you to explore the Trowbog national monument cave.