Nickelodeon All-Star Brawl 2

This can, and absolutely should, be done on the easiest difficulty. Character choice here probably doesn't matter much, but I chose a "heavy" character (Reptar, Plankton, Nigel, etc.) to make it less likely that an opponent would randomly knock me off a platform. The riskiest part of this is making sure you don't accidentally fall off a platform on a precarious stage. I tried to stay stationary on a decent sized platform and just do smash attacks over and over, charging them up while waiting for my opponent to come to me, and just smash them away over and over until I beat them. Same thing on bosses: just mash  on them over and over, staying as close to the middle of the map as possible to prevent accidentally falling or getting knocked off. It may take a little luck, but considering you have to beat Arcade with all 25 Brawlers, you'll have plenty of chances.