Age of Wonders: Planetfall (PC) - Achievement - Wololo
Permanently convert 30 units to your side
A simple enough method for this achievement is to use Psi-Fish Sirens purchased from their dwellings once you enter 'Friendship' state with them on any particular map. 'Eyes of Eternity' is a good MOD to use with them as this increases their range by 2 hexes.
Use their 'Enticing Bond' ability to draw a unit in close against its will and then use the 'Impregnate' ability when the enemy unit is next to your Siren. This results in them fleeing for 2 turns afterwards and then dying, resulting in a Psi-Fish Chrysalis spawning in it's place under your command. This counts towards converting a unit.
The first Kir'Ko campaign level in the Planetfall base game is your earliest opportunity to do this method if you're playing the campaign as Psi-Fish are present and you have the option to fight Vanguard troops. Please note, this can only be used against humanoid biped targets.