I'll definitively look into adding a goal for the ennemies who can reach the bottom of the screen as many people rightfully pointed out that this could add some goals other than not dying, i'm also planning to add a score or killcount to keep track of the player's performance (i still have to learn how to do that though)
The cooldown might indeed be a good solution to rebalance the attack and prevent spamming
Thanks for your feedback and helpful tips :)