Legendary ShotG 15

To get this achievement I used Lei Luo with the setup below, achieving a 372k damage in one hit.

Storm Blast – lvl 3
Shadow Knight – lvl 1
Voltage Surge – lvl 1
Eye of the Storm – lvl 1

Fulminous Zap – lvl 3
Bolt From the Blue – lvl 1

And I only had the first 3 talents (Sniper, Gifted Man, Thunderborn)

Weapons:
Golden Bow +17

Wait until the boss is close to show up, press LT and then shot before the Unique Ability ends.

https://youtu.be/fhhHuTDz5oQ

At the end of the video I show my setup.