First, in multiplayer, go to the outfitter and select Red Harlow as your character. For this achievement you do need to kill real players, NPC kills won't count. You can do this on free roam, but I'd recommend doing it in Legends and Killers Free for All Matches. There are boxes that refill your deadeye that should be always used, and plenty of enemies to get kills from.
Since in my opinion deadeye is the best way to kill enemies anyway if you got a slow, powerful weapon, boosting this is in my opinion unnecessary - you'll get this in the course of a few matches, which you'd probably do anyway. Just remember to play as Red :)