It's not so much an issue of the game being difficult, as much as the game just requires a ton of knowledge to progress, made even trickier by the inconsistent difficulty curves and weapon balance.
I was bored at one point and wrote up a big list of how I would balance weapons (primarily just through price changes) and adjust some aspects of the store and how you buy things aimed at making the difficulty curve a bit smoother.
Not really sure what to do with this list of changes now, but I did try to make them as simple as possible just in case someone is able to add them. My understanding is that the Dev moved on from H-games, which is totally fair, but if they or someone else could make some simple number changes as a band-aid fix for the game balance before moving on completely, I think it'd go a long way.