Kill stealing takes you pretty far in ffa. You only have so many cooldowns, bullets, etc, so you need to be economical and an easy way to achieve that is to let somebody else do the work then steel the kill at the last moment.

Getting better at headshots with Heroes like mcree can help, too. When I play mcree in FFA it takes far less effort to maintain the top spot then with somebody like tracer who's ability dependant. Ability dependence can get you in trouble in FFA because the fights basically never end. You can find yourself always on cooldown. That's where the "get gud" comes in because if you can land your main weapon shots reliably then you'll use your abilities less.

If you really want to see some intense FFA play some headshot only widowmaker FFA in the custom game browser. Holy shit it's so fast in those matches. I'm a trash widow but I I improved my ad crouch spam in 1 hour of play in there because without ad crouch spam you're instakilled by 2+ people at once!