i don't know if this will help, but i looked in my guidebook and counted the skulls until i reached the sixteenth one.
It is located in 2.5, Ballista City 2, inside a dungeon but just before you technically enter the catacombs. Take the ramp down to the obelisk. The corpse should be just past the obelisk (which will be on your right side as you walk past it).
If this is the right one, it looks like it would be easy to miss. It's tucked into a dead end corner of sorts, and the obelisk might detract attention from it.
You don't have to replay the entire game to get the achievement. Just replay the level you missed an item, find it, then replay the final battle at the end of the game. The achievement should pop. Or so i've been told.
If the lore is mixed up and not number dependent by skull tho, it probably won't do you any good. i never noticed if it is or not. If you are going to play it all again, this guide will tell you how many items on each level, so that you can keep count.
http://www.trueachievements.com/a151454/its-all-a-top-priority-achievement.htm?showguides=1