This achievement is missable and can be gotten at the Platypus Bataka minigame.
The achievement description is a little bit misleading.
You can use the Training Dummy at least once to get used on how to fight, just don't overuse it to get completely skilled. (max four times, the fifth will max out - afaik)
I used the Dummy once and still got the achievement. (Flawless Victory! ... on 2nd attempt :D)
To get the cheevo you have to act quick as there is a timer in form of a red "clock" in the middle of the stage and you have to move the rectangle like box to either Head, Torso or Legs to protect the targeted area.
To see which part of the body is targeted watch out for the arm of Goal in which direction she's pointing.
To unleash your platypus wrath you have to - after a successful defense - watch out for which part of the body Goal is protecting (one with the stick, one with her arm) and go for the unprotected part.
You have to this like six or seven times, depending on how much you've skilled even less.
I hope the Guide is useful and well to understand, as English is not my mother tongue :D
Happy hunting!
Edit: To see the Dummy you have to loose, of course. I do think that the first encounter is scripted and you will automatically loose - this has to be explored deeper (as I do not intend to play the game again :D)