Well the only way to level up your Black Entree level is to take part in Funfest Missions.
But to make it quicker get a 2nd DS / Friend to take part in the mission as well.
To do this get your friend to click on the tab at the bottom of the C-Gear and as you take part in a mission it will ask your friend wether he wants to join.
This will raise your level up dramatically.
Plus doing better in a mission will raise your level up even higher.