One of the toughest achievements in the game, definitely dont try on harder than easy difficulty.
The first chance to do this would be on the University mission. I would recommend you though to wait for the next one (major or minor), as on the University you will face some very tough situations.
The objective is not to shoot any weapons or to use a grenade - duh :) This means that you have to order your guys to do all the dirty jobs, while you are supporting them with melee attacks and your abilities. Against the lesser enemies, you might even get away while completely being in the background, but if you meet Mutons or Sectoids, you have to get close to the fight. Which means, that you not only support them with Lift and Heal, but also keep on actively resurrect them as they will die. A lot.
Whenever you can afford, order you men to retreat to cover and wait for the abilities to recharge. I did it with a Commando and a Recon guy as their abilities seemed to fit the most.