I found the writing to be unclear from guildwiki and decided to do my own write-up for others trying to explore the Dune of Despair and post a map of it.
One of the biggest problem is the timer on this mission that prevents you from exploring the Dunes of Despair in one session. The key is to prevent the start of the timer. Another issue I had was the entrance to access the lower right region of the map (point D). On many previous attempts I thought the access to this lower right region is found somewhere in the tower at point C.
First off, I did this map with henchmen (bring casters who can deal range damages). My warrior/necro had the Necrotic Traversal skill so I can teleport to the corpse. I had the two running skills (Sprint and Charge) for continuous running.
At this point open the attached map and follow the instructions below.
1) Use long bow to target the skeleton warrior or boss at point A. Use Necrotic Traversal once you killed the skeleton warrior or boss at point A.
2) Run through the drawbridge at point B. The drawbridge appears to be up but it's a mirage.
3) Run along point F to expose this region.
4) Explore point G and point E. I had these areas explored on a previous trip.
5) At point C is another mirage with the drawbridge up: run through it to explore the tower.
6) Point D entrance is mark by large rib bones. You can run to your heart's content once you get into this area.

Should you have any questions feel free to make a post.