During natural gameplay early in the game, you will come up to a section where a destructible block in front of some tall grass will cause you to unwittingly reveal Rampelkimpers as a way to introduce the player to the aspect that the character is hiding in the game with upgrades that you can purchase.
But it seems that simply revealing Rampelkimpers hiding in the grass counts as an Upgrade, regardless if you buy the upgrade he is selling or not.
To set yourself up to earn this achievement, use a charged up bow and arrow shot to destroy the block, then bypass the grass where Rampelkimpers is hiding without slashing it with your sword.
Continue to play the game without purchasing anything from the vendors. Achievement will unlock when picking up the fang to end the game.