I think the main problem is that the player character can only move in 4 directions but the monsters can go in any direction, the effect is that in most cases you can't hit anything because the monsters are coming in at an angle.
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The player should be moving in all directions, as the monsters do. If I have understood correctly, the problem lies with the player's attack, which has a tiny collision and can miss a lot. This is something completely true and you are right: we did not think about the attack and when we implemented it it was already too late to change it. I think a circular attack (full circle or mid circle) would have been better.
Also, the game was not clear enough. The green slime cannot be killed, so no matter how many times you hit it, it won't die.
Thank you a lot for the feedback, it helps us grow!