So, you want that co-op achievement, but don't have any friends to play with you? The solution is very simple.

Kill Nana.

That's right, every time you start up a level, simply whittle your partner down to 0 lives in whatever way you see fit, and you're free to complete the level all by your lonesome.

Happy Treasure Hunting >:D

(If you prefer to play as Nana, feel free to kill Tobe instead)