For this one, you have to be the player out of both teams to do the most healing in a game. I will provide some tips for both team healers.
For Plants, the Sunflower has 2 ways to heal teammates. First off is the LB move which is the heal beam that will attach to players close by upon activation. This one will be what you use the most as you can be mobile and still attacking while the beam is active. The other is the RB skill, which drops a potted plant that drops sunlight (health). This one, if placed well (like in a grouping hiding behind some tall-nuts), can be very helpful in making you the top healing player.
For the Zombies, the Scientist is the healer and he only has one way of healing. His RB skill is to drop a healing station that sprays out purple goo that heals teammates in the direct area. It can heal anyone in the area, and if place well can really provide a good amount of healing. The station can be destroyed by the plant team.
After the round is over, you'll see who received the medals for the round, which can be defined on the third tab after the sticker shop. So after getting the healing medal 3 times, this achievement will be yours.
In my opinion, this achievement is much easier to get on the plant team. The Sunflower has both a mobile and stationary healing skill which means you can be healing your team at all times.
I hope this guide helps anyone that is looking for some tips on achieving this goal.
Please comment and let me know if there are any other tips that should be added, or if I have made any mistakes in my information!
Also if you're going to vote negatively, I'd appreciate some feedback as to why.