The actual description for how to earn the achievement is as follows:
As Medusa deliver the killing blow to an enemy using Lacerate.
Medusa's lacerate is her third ability (
or 3). Medusa quickly slithers forward rooting and slashing the first enemy god she encounters. Lacerate itself does the following damage: 80/120/160/200/240 (+80% of your physical power). With that being said your best bet for netting this achievement is to either catch an enemy who has little health running away or weaken someone/petrify and follow up with lacerate.