About the enemies only shooting you when you're level with them: how about making it so that enemies only *become* aware of you when you're level with them, but then continue to be aware of you until you completely leave their sight?
I actually already implemented this mechanic in the early development, but it felt too hard and decided to scrap it. But the weapons and health balance was way off back then so I might give it another shot and see how it feels now
