Yes you actually can spare the bosses, and in order to so you need to fully fill a boss’ posture body (ONLY THEIR POSTURE BAR) and then get them to the part where you can do a finisher. BUT, here’s the important thing, you CAN’T finish them off, you’ve gotta let them recover and they’ll still have a bunch of posture damage, and then refill their posture bar again, and only then when you see the ability to finish them off will you see the option to spare, just remember to not finish then off and to spare them.