For this achievement you'll need to complete Stage 3 of Arcade Mode.
For the Arcade Mode of Stage 3 you'll be facing against Bark. If you want an easy going match you can choose Easy mode, with attack power +3, and only 1 barrier so you can easily overpower your opponent for an easy win.
Once you've defeated Bark you'll earn this 20G achievement.
The following video will depict this solution.