MultiVersus

There are two solid methods of doing this that are quick. One with another player and one without.

Solo Method: Go to customs from the main menu and change the mode to 1v1, set kills to win to 1 and then add a bot and set their difficulty to very easy. I usually went against iron giant because of his massive hitbox. Just kill him once, rematch, then kill him again. That will count as two wins. Then it will take you back to the main menu and now you rinse and repeat.

Partner method: This method is best for also going for things such as projectile kills (Need 200) and spike kills (Need 100) so that you can do them at the same time. 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. For projectile kills play as reindog and use his Neutral attack while facing your opponent and with them at the ledge and it will shoot them off the edge. Or use Shaggy and for him press down on the left thumbstick and press at the same time. This will pull out a sandwich. Now face your opponent who is on the ledge and use the same controls again to throw the sandwich at them.