This is part of the 3.5 update.
In Gems of War, a weapon gets an Affix on the 6th upgrade or higher. That means only rare (green) weapons or higher can get an affix.
To check a weapon, go to the weapon tab in the troops category and press RB to upgrade a weapon.
dev notes: You can get Ingots from:
Randomly from a PVP battle (this includes, Guild Wars and Arena)
PVP Tier and Ranked Rewards
Gem Chests (which contain Ultra-Rare ingots or better)
Weapon Bundles in the Shop
Raid Boss + Invasion Event Rewards and Shop
Upgrading ingots in the Soulforge (at a ratio of 10 to 1)
The easiest way to get this achievement is to farm PvP (any battle) with a fast farming team and win. Losing does not reward Ingots. It can be done in Casual as well.
Rare (green) weapons take 21 rare ingots, so between using the Soulforge to convert 10 common ingots to 1 rare or using rare ingots, you'll level it up quickly.
After that, use the weapon in a battle and the Achievement will unlock.
As of the Raid Event on July 2nd, you can buy up to Tier 5 (cost is 30 gems, 70 gems, 150 gems, 250 gems, 350 gems for a total of 850 gems) and get the first affix on a Legendary weapon without effort. Legendary Ingots are being sold at Tier 3, 4 and 5. Would I recommend it just for the achievement? Not really, but its an option. Its a decent amount of Legendary Ingots for any long term player though.