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Like others have said spamming space bar is a very powerful strategy. Another way to fix this issue is to have enemies or hazards that can hurt you when you're charging or to add a cooldown/energy bar mechanic to the charge attack itself. Another suggestion would be to improve the enemy kill feedback; it's very important to the overall feel of a SHMUP. If killing enemies feels satisfying (usually with a lot of large explosions) the game just feels more fun.