You must get the vortex grenade as part of the weapon DLC. It is an amazingly powerful grenade, so it is worth owning it. In the Pripyat map, there is a merry go round. Just make sure to hit some enemies here, at least 6 to get the trophy. It is fairly easy.
Hit at least 6 living Xenos with a Vortex Grenade in Pripyat amusement park