The enemies have too much health. The way to go about this is to either have an indication of their health or slowly introduce higher health enemies over time. I honestly thought the gun was broken XD
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Thanks for playing. Since enemies dont have healthbar , I think it will be confusing for player that enemies start to take more hit at next level. I give enemies much health so they can gather and attack you as one group, which increase difficulty if you cant take down most of them at the beginning of each level.