Using ER specifically in refence, there are several buffs that you both spend health to apply, and ones that activate at certain health thresholds. If you look up ER non-magic boss 1shots. First one that comes to mind is the seppuku one where you stab yourself.
Several weapons also play around with healing on hit, so you can keep yourself at low thresholds fairly easily.
Kolanaki@pawb.social 3 days ago
I did it in Ghost of Tsushima using that one sword set that doubled attacks when at or blow a certain health threshold (basically if the bar was red instead of white). But that isn’t a hard game to begin with.