This achievement can be unlocked over time, in both private and public matches.
It can be tracked by going into...
Milestone->battlemode->True between Demons
All demons have three sets of healing abilities. The first can be obtained right away by going into your load-outs, go to actions, and change the action set to the one that has the health ability.
Archvile: Action Set Summoner
Mancubus: Action Set Support
Marauder: Action Set Pressure
Pain Elemental: Action Set Summoner
Revenant: Action Set Defensive
This first healing ability only works in a small area and heals you and your teammate 100 health every second, for 8 seconds. You can select it using the D-pad in-game.
The second healing ability Team Heal can only be obtained on round 4, if one team wins 3 times in a row, then you will not have the option to unlock this ability, as the match would have concluded. This ability will heal 50% of your health and 25% of your teammate.
To get the most out of Team Heal use Bigger Bar, as it gives you an additional 500 points of health.
The less useful out of the three is Thieving Hand, this will return 50% of the damage from summoned demons back to you as health.