What determines low health for the Survivor achievement?
Solution 1:
Actually it's 10% health and under. I made a video to show you how to get 250 of them in a few minutes only, and all the requirements for getting the "survivor" message are listed in the description.
Solution 2:
It's below 5% of your maximum health.
I had a bug where the achievement was incorrectly awarded to me the first time where I survived a fight with low health (I just realized this now since you're asking about it, it seems like it wasn't added to the achievements list, though).
It said something like "Have under 5% health in a fight and survive".