This achievement is broken into a couple of steps.
STEP ONE - Tame a Sour Mallowolf
Once you hit Level 31 as a gardener you will have the Sour Mallowolf appear in your garden, although it will not visiting until you hit Level 32.
Once you hit Level 32, the Sour Mallowolf will visit your garden and started pestering your other pinatas. As with all Sour Pinatas, you are able to tame them and "evolve" them to a "normal breed".
To "tame" a Sour Mallowolf, it must east a Pigxie (which is created by cross-romancing a Swanana and a Rashberry; which also nets you an achievement). Once it has eaten the Pigxie, it will then become tame and a resident pinata.
STEP TWO - Send it after a Ruffian
This is the simple part really. First of all make sure you don't have a "Captian Cutless" ornament in your garden (this stops Ruffians visting) and when a Ruffian comes into your garden, select your newly tamed Mallowolf and direct it towards the Ruffian. If it doesn't go towards the Ruffian, simply re-direct it again.
The Mallowolf will scare the Ruffian aware and then the achievement will unlock.