the better your bug spray the quicker you can kill the beatles. just don't upgrade it so far that you don't have money to finish the level.

if you have the trading post and sherifs office you can go on the missions for more points. the trading posts ones are a little tough they took some thinking to complete. on the sheriff ones if you cant' do it with the sherif and 3 deputies build another station and hire more deputies then you will be able to do it. it also helps if every one is at full health and guns are upgraded. for each one you complete you get points and other stuff.