The easter egg on level 7 was hilarious. It's amazing how great efforts in game design can come together with music (read: full orchestra) to create a game as relevant today as it was in 2007. The details are astonishing.
Note on level 13: If you cross the border early on, the second half should be a breeze. The upgraded Para 98 also helps, as others have pointed out.
Muahaha xD seems like u gotta make this into a challenging game youreself to make it any fun:P I just came to lvl 10 without buying anything at all :PP