A nuker refers to a dedicated damage dealer, usually having a high crit rate, high crit damage, and hight attack. Genbo, for example, is a nuker, if you got him recently from the Amazon Prime drop. A word of warning is that nukers can be very squishy so often need support in the form of at least one healer or reviver if you need sustain over multiple rounds. 

A "nuker" is a high level champ that can drop an enemy or team (or at least cause massive damage) with one shot.  This is best doen with someone dropping defense right before they go.  Example: Each of your starters has an Area of Effect (AOE) attack, if you build them, whoever it is, with 100% Crit Rate and REALLY high Crit Damage they can be used as your "NUKER".  Just like farming, though, you'll find better as you go.  Now I mentioned dropping defense.  A farmable rare, Warmaiden, can be found in the deadlands, if you look hard enough.  She drops defense by 60%, allowing the nuker to do their job easier.  She would take her turn just before the nuker, meaning her speed is 1 or two notches faster.  

You're nuker doesn't have to be HP, but they do generally have a higher base HP and are easier to keep alive if the first attack doesn't kill all your enemies.

Building them-

let's start with the bottom row:

Rings-Whatever their damage is based on (DEF/ATK/HP)

Amulet-Needs to be Crit Damage.  Ideally with a substat of their DMG base

Banner-If they cast debuffs, use ACC, if not, use either RES or dmg base

Middle row:

Gauntlets-Crit Rate

Chestpiece-same as banner, always use % vs flat stat (i.e. DEF 5% instead of DEF 25)

Boots-SPD, always always SPD.  Sell anything that is not SPD

Weapons/Helmet/Shield-always flat stat, substats are what you are looking for here, round out your Crit Rate and increase Crit DMG, DMG base is again key here, but Crit Rate and Crit DMG are more important.

DO NOT FOCUS ON SETS, STATS ARE MORE IMPORTANT