I ended up becoming a literal death machine with max shuriken, cooldown and attack levels. I even had near max shotgun and sawblade levels for good measure. I carved a path to the chopper with machine-gun shuriken shots, so that was fun.
And then the Katana run - or, as I like to call it, the "lawnmower run" thanks to max cooldown and area, plus a few sawblades. Overall, a great little game for some quick fun.