Risky Blow (オーガーぎり, Ougaa Giri?) "Ogre Slice", Will cause a critical strike if hits, low to-hit rate.
Risky Blow costs 0 AP to use and is taught by Bunyan after being his apprentice for 2 levels. It is also possible to learn it from several bosses/monsters.
Due to its low hit rate, this skill is easily ignored, but should not dissuade you from taking Bunyan as a master.