This is where I'm at too. Like, I'm not complaining, because it's fun just to do a few levels in co-op, but if they put down some more characters, or some kind of side story to fit more villains in, I'd likely buy it. It's just pure, unadulterated fun, and I want more of it.
I did Story Mode on Gnarly difficulty, only to realise the achievement specified Arcade Mode. Woopsies! Got some practice in, I guess, and some more level ups.
@Hero-of-Time, are there a limited number of continues in Arcade? Because I'm not the best at these kind of games, and depending on how lenient it is, I might not be able to pull it off.