My Item Level Is Higher than Yours

This achievement is earned by augmenting the power of your level 100 followers by using items such as Armor Enhancement Token and Weapon Enhancement Token. These can be acquired by doing missions or opening crates at your salvage yard. They upgrade item levels by as low as 3 for uncommon, or up to 9 for epic, depending on the quality of your token.

This process is useful because it increases the chance that you will succeed on missions with higher iLvl followers, and that it can unlock more rewarding, albeit dangerous, missions. Once you reach iLvl 635, you unlock pre-raid missions that reward items such as Abrogator Stone, which is used in your legendary ring series with Khadgar.

Once you reach level 650, you can unlock raid missions that reward Cache of Highmaul Treasures, which has a chance to give you a raid quality item.

Good luck fellow garrison lovers!

EDIT: The pre-raid missions are unlocked at iLvl 630, my mistake.

As of the time I'm posting this, the requirement for unlocking Highmaul missions is having 3 followers at 645. As soon as you do that the first Highmaul mission will unlock. Also, killing any 15 normal bosses unlocks Heroic rewards and killing any 15 heroic bosses unlocks Mythic rewards.