I think this is doable if there are not unexpected delays. The skytrain is on the level above departures (arrival is lowest level). To get there, I've always taking a set of escalators up that are accessible from inside the terminal. I've found that not all of the exit doors have the set of escalators going up so you may need to follow some signs and talk. Once arriving, I would immediately go up one level as the departures level is less hectic than arrivals in the afternoon.

DUS essentially one big terminal with two ends and the Skytrain makes a stop in each area (although not necessary at the end of the terminal).