As Atlas, damage 3 enemy gods with the travel portion of Gamma-Ray Burst with a single ultimate cast. This does not include gods in the initial damage area when it begins.
Gamma-Ray Burst is Atlas' ultimate ability. When activated, a beam will come from the sky and hit enemies in the target area. After 5 seconds, or upon re-fire, the beam will focus in power and begin travelling across the battleground. It'll travel in the direction you're looking just as the movement starts. For this achievement, you must hit (not kill) 3 enemy gods with this travelling section. Any enemies hit by the initial burst and not the travelling section, will not count.
This can be attempted in any mode. Arena is good as enemies tend to group up quite a lot, so you'll be able to throw the beam at them in groups. A laned mode would also work too, especially late game Conquest or Assault, as you can fire up a lane or towards one of the big objectives, such as Gold Fury or Fire Giant.
Hitting three enemy gods with the travel section, will net you this achievement