first: get magnets. they will help you collect more money. you need about 4 fully upgraded to have the full effect. second: get the lasers. if you have a lot of them, spread out, and fully upgraded they will kill most of the enemies in about 1 second.
tip: if you want to get the "hard" badge it is a lot easier to finish the game and get upgrades to the max than to keep doing a level over and over. the pulse works the best for the bosses
FYI: If you are stupid and throw your plane backwards, then minimize the browser and bring it back up. Just try to refrain from repeating the mistakes of million of other players.
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