Well first make sure your chracters have all the best (currently) equiped items use the custimize to make sure that they are their best. And have the best Licenses Points(LP)
Now go and battle the easier monster such as the cacti and the wovles. If they appear to be no challenge try a little bit stronger of a monster. Battling these monsters will boost chracters expierence and LP.
When your people get around level 20 try and push forward through the deseret.
*Also remeber not to attack elementals, or any monster with a red name, or even a yellow. However a yellow is beatable but red is difficult. *

**Since you are heading into the desert make sure to take the path leading to the blue crystal to benefit you in the future**