In order to get this achievement, you will need to finish the second stage: Mermaid Falls. First you will have to fight your way through the level and then beat the boss.
Once you kill the boss, the achievement will pop.
Interesting side note: in the middle of the stage you gonna encounter a seller, giving you the opportunity to buy some stuff (and get a cheevo).
Here is a video for the fight:
(Credit goes to Kriss)
My personal tipps:
- make sure you entire the fight with enough life/hearts (or carry some potions/fruits/...)
- you can play the first stage a few times to get enough gold. visit the item shop in the main street and buy some upgrades (more damage from your attacks and less taken damage)
Going through this level will net you those cheevos:
(you have to buy something!)
- having to possibility to dodge (LT-Button) also helps for the final boss
If you start from scratch, it should take you about an hour to reach this point.