I am very nearly fully upgraded in Zombie Gunship, but I can't seem to improve any further. I get to about 750 zombies consistently.
One major tip: always engage the zombies at as far a distance from your bunker's entrance as possible - especially the extra-large zombies. Even if you don't kill them in one go, softening them up before they get too close is critical. I consider the entire game to be an exercise in "time management", similar to an airport simulator!
I don't know if you realize this, but the number of stars in the top right corner of an objective will correspondingly fill in an equal number of stars on the "Progress" page, once that Objective is finished (by either completing its goal or paying the required "ransom" in coins). Getting enough stars to be promoted is critical, since each rank increases the score multiplier, which results in your being able to earn more coins/round.