Off And On Again

The fastest and most efficient way to get 100 games played in multiplayer mode is to use the Invasion mode (the bottom left mode in the multiplayer setup screen) and the best part is that you only need 1 controller for this method. Make sure to set it to 1 life and there's only one arena type for this mode so start it up. Once it begins, you want to quickly spam the pause button every so often until you see a bullet come out from one of the space invader style enemies. By pausing the game with the bullet onscreen, you can have time to react for when you unpause and attempt to stand under it to die so that the match ends. Try either standing on the left half or the right half of the screen as one of those two sides will be easier to make your way to. While the bullets are in random spots, I found that waiting in the middle would usually not give me enough time to get to either end, so I'd say pick either left or right and stick with it. You won't always be able to get to the bullet fast enough but using this pause spam trick, you should get pretty good results. The achievement should be way quicker using this mode and method!

You will need a second controller to do this achievement.

Go into multiplayer, select any of the game modes and maps (personally I found elimination to be the best as you can afk most of the achievement). Completing any of the matches will add towards the achievement.

If you select Elimination, you will start with 100 points and gain +5 for a kill and -10 for dying (from being shot). However, as the match progresses, the points of everyone will start decreasing. If the points of the players reach zero they will lose. Using this, if you afk in the match, it will count as competing in the arena even if both players die simultaneously. You will only need to come back to the controller to hit rematch once the match is finished. (There is no audio cue for matches ending)

Each match this way takes approx 70 seconds from selecting rematch. Which totals to just under 2 hours this way. If you actively wish to shorten this time, you can use the controllers to make one character lose more points, in which case, each match will take approx 35-40 seconds.