Syndicate

Easily attainable with 'Squad Heal'.

Unlock 'Squad Heal' and set it as your primary research. Once it has been fully researched, be sure to equip it as one of your perks.

Playing on 'Hard' difficulty, your teammates take greater amount of damage when shot. Once everyone is below the 50% mark, use 'Squad Heal'

or...

Boosting method:
If you want to boost this. Load up the map Mozambique on Hard.

You and your group make your way to the security booth with the turret above it. Be sure to clear out all enemies beforehand. Once you reach the security booth, do not destroy or disable the turret. Just run past it and take cover inside the booth.

As SoiledDoughnut mentioned below, friendly fire will not work, so you'll need to take damage from the turret. You can heal your teammates individually, (does not have to be all 3 at once.) As long as you heal them within the 1 minute mark.

Now take turns walking out of the booth, taking damage from the turret, while 1 person does all the healing. Wait for the health bar to go below 50%, before you heal. Again, this has to be within 1 minute, so just quickly rotate taking damage until the healer unlocks it, then switch.

...or you can use the more random approach by spamming '' to heal your teammates, but be sure you're playing on 'Hard' difficulty.

Good luck.