For those looking to do this, it is easiest to do it with a friend or two in an online room.
In the Injustice 2 menus, go to Online then Rooms. Then opt to create a room and name it what you want. Tell your friend(s) to join the room or invite them if you can and once they are in the room, challenge them to a King of the Hill match. You can limit the number of participants down to three so if you have 2 buddies it should be no issue getting the achievement. Just let the challenger win and, if there are more than two of you in the room, you can work on getting the "I Voted" achievement too.