Solasta: Crown of the Magister

This achievement is based completely on RNG. When a character drops to 0 hit points, they make death saving throws automatically each turn as long as you do not stabilize them or finish the combat. A roll of 9 or less is a failure, while 10 or higher is a success. A 1 is two failures, while a 20 is two successes. No modifiers are applied to this roll, and it may take some time to happen naturally. Once a character gets 2 failures and 3 successes, the achievement will pop.

Edit: Turns out once your character gets to 2 failed saving throws, you can stabilize them normally, and the achievement will still pop. This is much easier than the above solution, cos RNG is not something you want to depend on.