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.
Hint : When fire from the double shot (slower) weapon hits an enemy, it re-charges the faster single shot weapon. To keep the single shot re-charged it is possible to fire both weapons at once by tapping both fire buttons, maintaining charge, giving a higher rate of fire and more chance of hitting enemies.
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.