Hint : To get a high score, if you do not miss any shots your bonus score will keep on doubling to 10,000 then reset to 000. When at 10,000 miss a shot then bonus score will revert to 5,000.
Bug : In the challenge stage, if you shoot all 40 aliens you normally get a bonus charge of 60. Sometimes a red hydra comes out during this stage. If you shoot it and all 40 aliens, you get a total of 41. The game seems to fail the test of 'hits=40' and only gives you a bonus charge of 41.