MultiVersus

All you need is a friend or second console plus no jumping needed. Quick and simple. So get the 2nd character to the tip of the tip on the edge than use Shaggy to Power stomp (look in move list)
. Done.
This will knock off the second player without them bouncing which will count.
AGAIN MAKE SURE THEY DONT BOUNCE THEY NEED TO GO STRAIGHT TO THE BOTTOM.
It will only take 9 games to do a 100 if you do it right every single time because the 1st round you win 4 RingOuts -2nd round you lose but get 3 RO. And 3rd round you win with 4 RO.

This method will have both a solo and partner method. The first two methods are assuming you are also trying to win your 300 matchmade games.

Partner method: Go to customs, 1v1, kills to win = 1. Communicate how you want to win to your partner and rotate wins each match. For spikes have your teammate go off the edge and slide down the wall, now just go to them, aim at them while jumping in the air and press down on the left stick and at the same time and try your best to hit them. If you connect your hit they will go flying down and count as a spike kill when they die. I found that the best character for me personally to do this was with Finn.

Solo Method: Same custom settings but with a bot added on very easy difficulty. Now you have to try and knock them off of the map and downspike them by using down on the left stick with

If you are not trying to get the matchmade wins at the same time just set the kills to 4 or whatever the highest setting is and get 4 kills per round or whatever the highest is.