Mind Control unlocks pretty late into the campaign. Rank 8 to be precise. As soon as you have it available start using it. Always go for the tough enemies since standard ones will get annihilated by their comrades right away. The cooldown is murder for this one.
At rank 9 you have another ability unlocked named Mind Merge. This will improve damage and toughness of controlled aliens.
You should also weaken some smaller enemies so the controlled ally only has to knick them off.
Most fun thing to do is to control a Muton and watch him nuke his whole group
.