You don't get any time for upgrading between the enemies spawning. enemies have too much health. With the amount of clicking required to take the enemies down, the starting weapon should be on auto-fire.
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Thanks for your feedback. The enemies pause when you open the chest. I wanted to make a wave spawner but my game was too ambitious for the time frame so i didnt have the time. I guess the point is that you start to notice your gun becoming more powerful. The strongest enemy need 9 hits when you first start, which imo isnt too much considering you can buy your first upgrade after 4 enemies.