This achievement is most easily done with a hunter or melee assassin character. You need to wait for an enemy god to be very low health, or better yet, running away with low health, then move in for the kill. It does not have to be a basic attack, you can use your abilities and I would recommend it. An ultimate (Y) attack can be very good for this, such as Medusa with her Petrify or Ra with his Celestial Beam or Searing Pain. Basically anything that hits from far away and does decent damage.