Cadence gear is basically anything you make from enemy parts. If the item description mentions the thing is made from or inspired by enemies, you know it's a Cadence item.
I would suggest making the weapon and armor for the same class, then deploying a vanguard of 5 of only that class (or variants thereof) equipped with the gear and an item. If any of them have an extra item slot, make sure you fill that as well.
The achievement unlocks as soon as the battle starts, so feel free to save beforehand and just load when you're done.