OP is looking for "the rest." After you clear out all the downward, straight-forward Elites as you descend (normally), you'll be facing the Banshees with the cosmos to your right. After taking these guys out, all would seem clear. But if you go hard left, there's a doorway that leads into a narrow hallway. Following this will take you to a group of nasties. Clear them out, and you've got the achievement. I know this, because my sop in the day was to just drop down (from level 2), and Rambo it into that exact hallway. (The opposite side from which the "standard" approach leaves one.) Any way you cut it, it's a tough fight.