if i was you i would work with a waves system, the business girl should appear on this waves intervals
the gameplay is fun, but if you miss the business girl and run out of bullets it's impossible to survive, one easy solution would be some zombies dropping munition when killed, think on a cat playing with a ball of yarn, if the ball is unreachable, all the fun ends, the survival in this game is like this ball, you need bullets to kill and survive.
something i really liked in this game is the music!