Efficient magical in combat healing in 5e is all about Healing Word, you wait until your allies are dropped to 0 than you heal them up with a bonus action.
Why? A few good reasons.
There is no penalty for standing up besides a bit of reduced movement, there isn't a threat of an attack of opportunity for instance.
All the damage done to your companion that went over reducing them to 0 h.p is wasted damage you don't have to heal.
The healer doesn't have to be adjacent to the downed person with healing word, and all it takes is a bonus action to cast it so the healer can spend their action doing damage which in turn ends the combat sooner and reduces the amount of healing needed.
So don't heal the barbarian or anyone else until they drop to 0 in combat, it wastes actions and when they get taken down below 0 it is like all that extra damage is a waste, keep your allies hovering at or near 0 h.p.
Now as soon as the combat is over get them back up to max h.p as fast as you can with Healer feat, your sack of goodberries, or cure wounds spells.