No %damage will be used, it all comes from Chroma's Vex Armor.

This weapon covers seven elements: Impact, Puncture, Slash, Rad, Viral, Blast, and Electric.

Only 1 forma is needed to fit this entire build! For this reason;

Consider making a second, Magnetic/Rad/Blast/Toxin one for your Necramech, though it's not needed.

IPS Gas Mag Rad Primary

Impact Blast Toxin Pistol

IPS Corrosive Viral Cold Melee

With these weapons combined, we cover every element at least once.

Only change the shield element if is weak to electric before your archgun is out.

Additional coverage can be had by equipping your Necramech with a different archgun.