After grinding Card-Jitsu for 7+ hours and facing many frustrating in-game glitches, I have finally received the black belt and beat Sensei.
Unfortunately, due to Card-Jitsu still having bugs, I have not yet received the Ninja Mask for beating Sensei as a black belt.
I will keep you all updated!
FIGHT THE GOOD FIGHT!
~ Gallant, RPF Second in Command