The actual description for earning the achievement is as follows:
As Medusa stun three or more enemy gods using Petrify.
This one is quite straight forward. Medusa's ultimate (
or 4) is her petrify attack. The attack itself is a cone shaped ability that emanates from her face that turns enemies looking at her to stone. In order for you to successfully 'stun' someone they must be looking directly at you or turned to the side still somewhat facing you. If they are turned away the ability will only slow and not stun.
The single best combo for earning this achievement is to pair up with an Athena and wait for a three + man taunt. The taunt forces all people to look at Athena giving you a golden opportunity to unleash a petrify and net the achievement.