The details of this achievement don't match steam's. For this you need to Chickenify 20 enemies (not a dire wolf).
Once you get a Chickenify staff, just keep using it on enemies until it's affected 20, though note if you use it on an enemy whose health is above the max for the staff it won't work, but will still use up your mana.
Enemies whose health should be low enough for even the most basic staff will be peasants, so grinding at one of their spawning points should make this easy.