Hm... Not sure if I understnad what you're asking. Trent will stay where he is, Jon is just stupid. In your run, Jon wasn't perfect, and Trent kill was slow. You have to shoot while turning for last objective. In your video, you stopped for a bit before shooting. About Jon, just be patient and hope he isn't rolling while you shoot at him.
Edit: when you start objective 3, your timer has to be sub 6 or 6.0x at most, since you need to kill Trent in under 7.0 as most. Timer in your video is a bit hard to read, but you can still tell that your Magnum did shot in 7.xx.