Riolu does not learn "Aura Sphere", as it is not in its learnset. To get Riolu to learn "Aura Sphere", you must evolve it into Lucario. Lucario learns Aura Sphere sometime in level 30-40. If you are higher than that, simply go to Electivire Link Shop
Aura Sphrere would be lucario's best move. Most powerful. Pokemon Master & Lucaio Biologist ~Bolt the lucario
None. A sphere is pretty much a 3D circle