25 yards isn't really a face-off...it's 75 feet...that's some distance. I'd try to run.
If I could PICK a gun and a target, then obviously something like .454, .50AE, .500 S&W, or .44Mag. If I could pick a shot, then definitely center-mass heart. I don't care what kind of organism you are, a direct hit to the heart will drop you within a second or two, you may not be dead, but loss of consciousness due to blood loss will be instant.
But considering reality, it would be my 1911 with .45 FMJ, fired at his chest, hitting him in some vital organ. Then I would hope to God I can avoid getting hacked up while I run around and wait for him to bleed out.
Gorey scenarios, but what the hell, it's what the internet's for.