Location: After you defeat the Amazonian Worm sub-boss in Chapter 9, follow the way out of the tunnels and then immediately follow the path to the right. When you get to the top of a small hill, you'll find a dead ninja holding the Flails.
To get any of the Weapon Master achievements, you need to complete a playthrough of the game using only a single melee weapon. You can use ninpo and ranged weapons, but hitting another enemy or even blocking an incoming attack with another melee weapon will invalidate the achievement. See the Road Map for two different strategies for unlocking these eight achievements.