The Shinobi job isn't a job that requires you to attain certain job levels. It is actually a Ninja/Samurai/Lancer/Geomancer combo. Here is how to set up a Unit as a Shinobi:

Ninja Job
Jump (Lancer)
Blade Grasp (Samurai)
Attack UP (Geomancer)
Ignore Height (Lancer)

For best results, use the Easiest Job Mastering Method to obtain the required skills for a Shinobi much quicker.