It was only mandatory on a handful of bosses, either ones that had you working to expose their weakpoint anyway, like the weird mollusk thing in the water area, or against the Chozo warriors, and they'd automatically put you in the finisher cinematic when their HP was low enough.
It was only mandatory on a handful of bosses, either ones that had you working to expose their weakpoint anyway, like the weird mollusk thing in the water area, or against the Chozo warriors, and they'd automatically put you in the finisher cinematic when their HP was low enough.