To get this, you need to take any Sensei Skylander and bring them to the level cap of 20.
This seems like a daunting grind, except for one thing: The arena. In the arena, you can face wave after wave of experience granting enemies.
Simply keep entering into the arena and killing everything that moves. Even if you fail a particular round, you still have all the experience and gold you earned. Playing on hard (which gives a 25% exp boost), I was able to bring a sensei up to the level cap in a dozen rounds.