New High Score in Normal mode: 167880
To kill the flying saucer/worm put out at least 6 flame throwers fully upgraded then set down a lot of land mines. Don't place the land mines in intersections (if you're using a criss-cross pattern), place them on all straight pathways. I've killed that thing many times using this method. It also helps to have about 6 machine cun jeeps out as well. The boss waves (when the flying saucer/worm comes out) are 11, 20, 29, and 38. Level 47 is a swarm wave so all your defenxes need to be fully upgraded by then.
You can also use the stealth bomber to kill it if you don't want to spend all the money necessary for all the extra defenses.
Hope this helps.
