To get this achievement, you have to defeat any enemy using the Carrot weapon while wearing the Bunny Costume. The Carrot is a two-handed weapon dropped by enemies or acquired from shops in Act 4. You'll need to unlock the two-handed weapon category before you'll encounter this weapon.
To get the Bunny Costume, you have to complete Wildcard Spell
mode (for Remi) and Wildcard Weapon
mode (for Choux). Your weapon MUST LAND THE FINISHING BLOW. If an enemy is defeated by Lore’s bullets or an ultimate spell, you won’t unlock the achievement. To be safe, use buffing Ultimate Spells
instead like Rage to prevent Lore's spell from accidentally stealing the final blow from you.
How to get Remi's Bunny Costume
How to unlock the Bun! Bun! Achievement