It is always advisable to travel in convoy so that you can help each other. Vehicles have to be equipped with steel plates to get them out of sand traps. On softer sand the tyre pressure of the vehicles should be lowered to present a larger contact area and on harder ground the tyres should be pumped up again. If traveling at 80km/hr, you could cover the distance to Nouakchott with only one night in the desert.
There is also evidence that it is now possible to drive north from Mauritania to Morocco. On our way south we met a German and a Swiss person at Guergguarat, on the Moroccan side of the border who had just driven north from Mauritania and they had traveled north without a guide.
The border crossing at Rosso is a bit of a nightmare. Listen to no-one and don’t change too much money as you’ll be ripped off. You need UM100 or CFA500 for the boat as a foot passenger. Keep an eye on your stuff too.