For this achievement you'll need to recruit 5 or more Phoenix Guard in a single army as a High Elves faction. This can be done in both the Eye of the Vortex campaign and Mortal Empires campaign.
To recruit Phoenix Guards you'll need to have a lvl 5 city. Build the Shrine of Asuryan and upgrade it to the Sanctuary of Asuryan. Take an army with at least 5 empty slots and recruit 5 units of Phoenix Guard in the army.
When the army contains all five Phoenix Guard units, the achievement should unlock.