This achievement is missable, although not incredibly hard to attain. You must get this before entering the Hall of Echoes.
The Killer Bunny appears after you have killed around 30 bunnies and can appear anywhere. When the number is reached, there is a short cut scene and the Killer Bunny appears. As this bunny is level 22 which can make him quite tough right at the beginning, but if you chose an archer you can kite him and beat him in the Broken Valley.
An alternative is to wait until slightly later, kill a lot of rabbits in the Broken Valley as there are a lot of them, but wait until Orobas Fjords for the remainder. Saving regularly helps here as the number of bunnies seems to fluctuate slightly, so after every couple save and continue.
* Spoiler * Broken Valley finishes after Maxos temple, so either make sure you have killed the majority of the amount, or the full amount if you decide to kite the bunny before finishing said place.