when your chance of shooting is over,it would be better if the opponent would automatically play.rather than me hitting the "end turn"button,for his chance
if you are running a bit low on cash play level 5 again.stay on the southernmost part of the beach and let those spiders and the opponent fight.when the opponent dies collect the cash.
there could have been a target "air or ground" units button ,like for example you're using lots of artillery and barracks (which does not target air units)and have one archer which is attacking ground units the flying enemies could easily take advantage of it .