McCree's
Deadeye ultimate ability allows him to lock on to on-screen targets, shooting and killing all of them.
During High Noon, McCree will begin to lock-on to targets. Higher health targets take longer to lock on to, as High Noon builds at 200 health/second. You can only lock-on to targets that are in your line of sight, and a red skull signifies that pulling the trigger results in a killshot.
Build your ultimate and then try to flank the attacking enemy team. If you're able to get behind them and use the ability with enough enemies lock-on for the killshot, just pull the trigger and unlock the achievement. It is also advised to be on higher ground to look down on the battlefield in an open area to see more enemies at a time. Keep in mind that D.Va in her mech suit does not count as a kill.
If you'd like to learn more about McCree and this achievement, you can view this video: