I think you might have read that list backwards. It’s things your action character should not do/say.
As I remember, “one shot, one kill” has it’s origins in the kind of sniper work where a second shot would get the sniper killed. Applying this to any other situation in order to make your character sound more badass is a terrible idea, and horribly cliche.
Actually killing someone with a single gunshot is surprisingly unreliable. Bullets have a nasty habit of doing strange things. And, people have a surprising ability to survive individual gunshots.
So your character goes for their “one shot, one kill” mantra… and shoots a mook in the head. What they don’t realize is the bullet deflected off the mook’s skull… and then your character gets shot six or eight times in the back, when the mook gets back up. And, yes, this can happen.
The alternative is, your character goes for their “one shot, one kill“ signature style… and shoots the mook in the stomach. Which results in your character being killed in a hail of gunfire from the mook’s automatic weapon because while they’re bleeding to death, they’re not dead yet.
“One shot, one kill” is your character choosing to make their life more difficult for bragging rights. Which may sound fine, until physics decides to mess with them, and their egotism gets them (and everyone in their immediate vicinity) killed.
Yeah, don’t have characters spout “one shot, one kill” off, it’s just bad writing.