MultiVersus

Ring outs in this game are accomplished any time you knock an opponent out of the blast box. I have a couple different methods of doing this

Solo: Set up a custom game, 1v1, score to 4, add a very easy difficulty bot. You can also farm projectile kills while doing this by using Shaggy and pressing down on the left thumbstick and at the same time to summon a sandwich and then the same thing again while facing the opponent to throw it. Just keep getting kills until your achievement pops.

Partner method: Same idea but your boosting partner allows you to kill them.

4 player method: (This is my double KO method as well) Find 3 other players either from Xbox LFG or by creating a TA session. Using this method you can get all 4 players over 100 takedowns and their 100 Double KOs in about 45 minutes to an hour depending on co-ordination.
Go to customs:
Mode: Free for All
Score to win: Highest scorer
Time: 15 minutes
Maps: Any that have a ledge already open on both left and right. I used Sky arena
Start by establishing the order in which you want to have people get the double kills, then have 2 people walk over the right or left side ledge to where if they get hit once they will fall off the side and die. Have person 1 use any attack to hit those 2 off the side and as long as they both die within about 1-2 seconds of one another it will count as a double kill. Now have two people go to the same side as before again, and second person gets two kills. Keep doing this until each of the 4 people get a turn. Now start with person 1 and repeat. If you can co-ordinate this well enough, each person should be getting 2-3 turns each minute making this a total breeze.