Most important for this achievement is, that your Psi-Soldier has Psi-Inspiration as well as Mind Control and the Psi-Armor. You want your Psi-Soldier to have around 100 Will (more is better) with the Psi-Armor.
When you engage an Ethereal:
-Leave your Psi-Soldier out of sight of the Ethereal (even your Psi-Soldier can be mind-controlled and then you have to kill the Ethereal instead of mind controlling it yourself.)
-In the turn before you attempt the Mind Control, use your Psi-Inspiration so you get a +30 Will Buff.
-Then attempt the mind control.
TIP: You can also have a Support skill perk, "Combat Drugs". The smoke grenades of this Support add another 20 Will.