The way i got to level 30 (died at 33) is that i upgraded my gun first all the way to max (18). The quickest way to do that for me was to stand on at least one flag until you HAVE to move, when an enemy comes near, obviously you don't stand there and let him pound on you. The enemy you most want to avoid is the kamikaze ones, kill them asap. for the guys who fire fireballs, if you stand in between the two left books, they'll stand there shooting the books, not you, but you can sit there and shoot them, while standing on the flag. Use mines and stuff only when absolutely needed, as they take up time where you can be shooting things.
an easy way to win on levels where theres no tanks is to send out, as quickly as possible, a set of riflemen and infantry soldiers through the middle, and anymore sets of infantry (depending on how much money you have). This strategy makes for a quick win. On tank levels i set out a tank and a bazooka behind it, then more infantry, because one tank and one bazooka guy easily takes out one tank. and these all worked for me on hardened too.
 "Bunny Charm"){kind=icon}
 "Pyro"){kind=icon}
 "Warfare 1944"){kind=icon}
 "Red"){kind=icon}
 "GeoSketch"){kind=icon}
 "Ancient Origins{flying fish}"){kind=icon}
 "Sonny 2"){kind=icon}
 "Run"){kind=icon}
 "The Last Stand 2"){kind=icon}
 "Heavy Weapons"){kind=icon}
 "CycloManiacs"){kind=icon}