Main focus of the build is that you go around running around with Viessa second skill, apply frostbite and gain +26% firearm ATK.
You will need a Viessa that runs Absolute Zero and heavily goes into skill crit rate.
The effect of Absolute Zero makes it that whenever a skill hit a crit it applies frostbite this applies to the second skill ticks, so for every tick that crits you will apply frostbite and gain the bonus +26% firearm ATK.
You also run Chill Enhancement so that you can also trigger Frostbite from your bullets, you can change this to any element that the boss is weak to.
Running Concentration Priority together with Recycling Genius to get really good crit dmg boost.
Adding Weak Point Insight for a small but great weak point dmg, since Eternal Willpower has a 1x as default getting it to 1.2x adds a 20% dmg increase for bosses.
Rifling Reinforcment, Fire Rate Up, Better Insight, Better Concentration and Action and Reaction modules are default for any weapon.
A big bonus is that if you have Shot Focus descendant module on Viessa it add another +15% firearm ATK and -15% Skill power modifier. Since Viessa has really big skill power modifiers for all her skills 1k on first skill, 250 on second, 1.1k on third, 1.5k on forth and 350 on passive a -15 to all those numbers is completely ignored.