Nickelodeon All-Star Brawl (US)

As the description says, you need to beat Arcade mode with all non-DLC characters. This can be done on Very Easy difficulty, making this an absolute piece of cake. In Arcade mode on Very Easy, you get five lives and your opponent gets one, and you have unlimited continues. For every single fight, just use your  attack (which is pretty much the equivalent of a smash attack from Super Smash Bros) over and over again on your opponent. I would often just juggle them up in the air with  +  until they were weak enough to go flying off screen. Some levels are little more annoying than others, but considering you can die five times, you will be able to make short work of each opponent.

While there are a ton of Arcade mode runs needed, each run only consists of seven stages (i.e. seven opponents), and each fight only takes a minute or two with the strategy above. It was taking me 8-10 minutes per playthrough on Very Easy, so while you do have a lot of characters to clear it with, it won't take long at all.

Note that, along the way to clearing Arcade mode with all characters, you should make it a point to do one run on Very Hard to take care of Ultimate Tool of Destruction (20G) and save yourself an additional run. The only other Arcade mode achievement is for clearing a run without losing a life. You will likely do this naturally during many of your necessary runs if you use the strategy above.