Assuming your opponent uses a physical attack on desu then you use a physical attack on them. Desukan is a wall iirc so that would work pretty well.
If you bring out Archaeos, your opponent is most likely going to switch to something that can kill it / wall it pretty well. When you expect that Pokemon to attack Archaeos, switch out to Desukan so it will obtain Mummy. Then later in the game your opponent would have to decide between switching in their counter to Archaeos and removing your penalty or letting whatever Pokemon is out take a hit and possibly die.
This all assumes Mummy stays between switches, and if it doesn't, kinda screws the strategy over.
Also could be used with Regigigas quite well.