Could also have do X amount of healing ex: 1 billion total healing, not including overheals. Shields and stoneskins count as much as the damage they mitigate.

You could make it like kills with different tiers that have different titles...

ex: 1 million, 10 million, 50 million, 100 million, 500 million, 1 billion

1 million heals is on the upper end of what most people do in a game and that includes overheals, so you're looking at something like 2000+ games or so for the 1 billion title. The only way to really farm it is to get locked into a long deadlocked battle for a long time, but there it's kind of encouraging you to stick with your group and heal people.

I support the idea of healer achievements, literally every solo queue game I do I end up going healer because I know it'll be needed the most.