MultiVersus

Okay this is my first time posting a guide and its gonna be pretty specific but it works like a charm. I have no doubt I could do this achievement just playing but it would take forever so I figured out a work around. Here it goes:

Set up a custom match set to free for all with a 15 minute time limit. Go to map rotation and select Trophy's E.D.G.E. 2, set up 3 bots as arya stark and select your character as shaggy. (These are the character combinations I found easiest for this method) towards the end of this 15 minute round, like the last 3 or 4 minutes. The edges of the map where ringouts can happen will shrink. When this happens, spam y on your controller to do shaggys roundhouse kick. When doing this you will likely get a lot of double ringouts. In 3 matches of doing this I am up to 47 of 100 double ringouts and I had 0 beforehand. Sorry this was long, hope it helps :)

P.s- obviously setting the bots on easier difficulty will speed up your progress

Context

Simply said: Knocking two opponents off the screen in quick succession.

This can be done in PvP, PvE and even in Custom Games.

2 Player Boosting Method

To obtain this quickly, set up a Custom Game Lobby with a 2nd Player:

• Game Mode: 1VS1
• Ringouts: 4
• Map: Scooby's Haunted Mansion
• Fighter: Superman
• Target: Quinn or Finn ^{(Low HP / Not a Tank)}

Synopsis

What you want to do is hit your target into the air, jump after it, and hit it again.
Use Superman, since he has a special Air attack that throws the target out of the arena, and is available from the start.

Video

Step by step

1. Once In-game, move your target to the middle of the arena.
2. Place Superman close to your target.
3. Hit your target into the air using the charged attack Overhead Strike (Hold +)
4. Jump after your target. ( + )
5. Whilst mid-air, throw your target out of the arena with the Go Long! special. (spam + )
6. Land next to your target, which will respawn in the middle of the arena.
7. Repeat from step 3.

Once you've won the match, you can vote for a rematch.
This will make it a tiny bit quicker, and give you progression on

Thanks to

I noticed unexpected progression on this achievement whilst working on a different one, following a guide by SPOONYxBARD:
Solution for Distanced Damage Dealer In MultiVersus

His solution suggests using Very Easy AI, which might also work for this achievement?
He also suggested using Quinn for this. So if that one is unlocked, it'll make this even easier.

Added Benefits

Working on this achievement will also give you progression on:

Disclaimer

The Achievement tracker seems a bit 'delayed' in this game;
For example: Doing something 10x unlocked, when the 100x was still at 9%.

Also: This method seems to be 'exploiting' an in-game timer meant for double-kills, so it might not count all the time, every time.
If anyone figures out what triggers the double-kill counter exactly, I will update the guide, and give you a cookie! Or cake.