You can get this playing as the medium class and using the healing beam specialization.
Once in a match, when a teammate has damage you want to heal them by pressing
and b using
. Once you've healed 25000 you'll unlock the achievement.
The medium class has a gadget called Defibrillator you can use by pressing
and the corresponding direction on the
, once selected press
to actually use it. This revives instantly after its been charged but only brings them back with 1/4 health and can help speed this up.