This is an update to an old build I had for the normal [Trumna] specifically for use with [Zephyr]. The build is exactly the same as what you'd use on the normal one but the primary fire is much stronger due to the improved crit damage & more importantly the status chance.
Unlike past versions of this build this one is fit for use on any frame without needing modifications. The guide here is still focused on use with [Zephyr] though.

Since you're always going to be pulling enemies into your tornadoes Blast has quadratic damage scaling. This means that your damage output scales much higher the more enemies you have in them at once. 2 = x4, 3 =x9, 4 = x16, 5 = x25, 6= x36, 7 = x49, 8 = x64... you get what I'm saying. This is also why we use Primed Cryo Rounds + Hammer Shot instead of Serration + Rime Rounds. More Blast = faster kills. We then add Rifle Elementalist to make all our Blast deal 90% more damage, which means each blast proc is dealing 570% of the damage the hit that caused it did.

Zephyr also passively gains +150% crit chance while in the air, and damage put into her tornadoes has an extra +200% crit damage added to its multiplier.
This means the Trumna's primary fire reaches 108% crit chance, giving it a 92% chance to deal 11.5x damage and an 8% chance to deal 22x damage instead.
The alt fire reaches 171% crit chance, giving it a % chance to deal 11.5x damage and a 71% chance to deal 22x damage instead.

You could use [Vigilante Fervor] instead of [Primed Shred] since we aren't really using the Punch Through it gives most of the time anyway. It would give you a small chance to upgrade your crits to the next tier as well, but I don't think you'll be having much trouble nuking hordes as it is. Always go for Fervor if you don't have [Primed Shred].

A Primed Bane mod would give more damage than Elementalist even to the blasts but it won't help against acolytes and Banes are annoying to work with.

Rivens are currently unslottable without Fire Rate due to low disposition. The only mod you can drop for a riven without losing power is [Primed Shred] so one of the stats NEEDS to be Fire Rate. As for the others I'd go for either Status Chance (if you really want it to be cracked on [Zephyr] specifically), Multishot (the best universally good choice), Crit Chance (3 bar no negative min roll will give you about 22% chance to t2 crit with primary, 93% with alt fire), or Damage (scales poorly with the arcane but makes stacking it up initially a lot easier).

Exilus mods are pretty useless for this but you probably want to have [Stabilizer] on this thing if you're using any other frame and not just shooting huge tornadoes.