Don't worry, that is exactly what I was talking about when I said I would "fix" things and "lower" the difficulty. I know the issue right even before I publicized the project. Gameplay is too much luck dependent right now.
To be honest, the 300 zombies goal isn't supposed to be achievable at normal circumstances at this point, on this current version. Me the developer never cleared it even once when I was testing this, and I expected the players to also give up very early since I already told them via dialogue that it is just "extra" content and no more unlockables after the first H-scene. But apparently, most players have the patience to keep going.
Well mistakes learned.
It really surprised me when people said they beat it, I was literally like "wtf how?!".
Since the project is currently at, I would consider, prototype-phase, the total gameplay feel might change on future versions but I assure you, I would STILL be incorporating the same a challenging difficulty (but a lot more fair), just because that's how I like my games to be and I don't really mind losing some people's interest over this. Like what they say, you win some you lose some.
I never thought of warning the players when enemies are incoming outside the boundaries, that is indeed a great idea!
This comment reassured me of many things, thank you very much for sharing your thoughts!