Marvel Ultimate Alliance

For this achievement, you will need at least one other player online or one controller with a Gamertag locally. It is much easier to do on your own with a second controller, but that is up to you. There are two spots that are great for this. One will require lots of reloading of old saves, the other you can basically do in one sitting. Either takes about the same amount of time (60-90 minutes).

I personally used the Sepulcher method. Extract to this location while you are in arcade mode (make sure you didn't select cooperative mode on accident), and then jump off the cliff with the second controller. With your main profile, you will need to get a minimum of four kills for the game to register that the level has actually been played. Except the lone fire giant, all the enemies here can be killed in one punch. There is also ample opportunity to throw them off the ledge to work on the "Widowmaker" achievement. Return to the extraction point and use the portal nearby to exit. You should get a results screen, and this counts as one win. After you spawn on the other side, use the portal again to return, and repeat this process 99 more times.

The other method is on Wrenchworks. In this method, you will have to save the game outside of Omega Base and kill the four enemies there. Head to the end of the level and exit for one win, then use the doorway to return and kill the four enemies you ran past near the exit; leave again for a second win. At this time, you'll need to reload the save you made and repeat this process. I don't like this method due to all the saving and reloading, plus the enemies take longer to kill. However, this has been the preferred method since the original game came out, so a ton of people have used it with success.