There is a much simpler way to get the healer achievement that only takes 1 round per person. We just did this in a 4v4 match (Myself, Lumberjay85, Eos Creon, WhiteneggXI, Cynicgods, Jimmy Noe, and 2 2nd accounts).
We had the person going for the achievement on Lineage, buy tree of life, and everyone on Lineage used a Human. Someone from RNA came over and did 3 headshots with a pistol in all 4 players on Lineage. Drop a tree of life (only 1 person!) and everyone fully heals. Now repeat the process for a 2nd set of headshots and tree of life to heal.
After everyone has fully healed twice, RNA kills everyone on Lineage (the team with the player going for healer) to end the round. The player going for the achievement had a 1.0 healing ratio in the score board. While the 2nd round was loading, we did a server quit, and the achievement unlocked.
The damage must be dealt by the opposing team, team damage does not work (we attempted that first).
This was confirmed by someone who has used Tree of Life many times over many sessions, and someone who has never used it and was new to the game. It is *NOT* an achievement received by some amount of lifetime healing, and it is not tied directly to the ratio of healing listed in the scoreboard.
Follow the above steps, and you to can be the proud owner of this achievement in just a few minutes!