Nickelodeon All-Star Brawl 2

From the main menu, pick Battle, and then set up a match where you're in with three level 9 CPUs. Hold  to open up the rule set, and make sure to enable teams. What you should also do to make this as easy as possible is turn on friendly fire (so the three CPUs can hurt each other), and set it to single stock, so as soon as someone dies, they're eliminated. Then, make sure you have yourself on a team alone, and the three CPUs on a team together.

For stage, pick any with small platforms over a pit, such as the Aquarium or Rocko's House. I tried out different CPU characters and didn't notice any real difference in behavior. For whatever reason, the CPUs have a tendancy to get caught up around the pit between two platforms if you stand on the closest edge. For example, in the Aquarium stage, if they're all on the left platform on the water, go on the right platform, but stand on the leftmost part of the platform. Their programming makes them want to jump to your platform to attack you, but they want to land first, and they can't because you're standing there. This makes them get all messed up, and sometimes just fall in the central pit for no reason. Sometimes they jump over you and get across, but you can be charging a smash attack at all times to be ready in case that happens. Often, they end up hitting each other while trying to recover, which is just as helpful. A little luck might be required here, but with them all struggling on one side of you, just keep charging smash attacks in their direction and hit them whenever they come close.

I ended up having one kill themselves in the middle pit, and the other two I just kept smashing as they got close to me until I sent them off the screen. It's not so bad once you can get them all on the same side of you and keep smashing them. Win the match to unlock this achievement, and if you're playing with a single stock you'll also unavoidably unlock Smashing! (10G) when you win.