For this achievement, you have to defeat ANY enemy using the Leeks weapon while wearing the Duck Costume. 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.
Leeks is a dual blades weapon so you can't get it until you have unlocked that weapon category. Once you have access to Dual Blades weapons, you have to either wait for it to drop from enemies in Act 4 or get it randomly from the shop.