Rurouni Kenshin

Sanosuke's Secret Training: The Challenge of Anji, the Destroyer (2x11)

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Sanosuke gets lost in a forest. He finds a monk there that goes by the name of Anji. Anji gives Sanosuke some food. Sanosuke ends up convincing Anji to teach him how to use the Futae no Kiwami, a very destructive punch (so destructive that it can completely pulverize huge boulders into dust with a single punch). Sanosuke has one week to learn it or Anji will kill him. Sanosuke finally learns it, and then leaves for Kyoto.