Game is doable solo. I’ve done all non-multiplayer trophies playing only with AI, and it wasn’t that hard. The reason why people are having problems is because the weapon crafting system has hidden gacha mechanics that decide how damage scales across weapon levels. If you are not aware of this system then you will upgrade the wrong weapon and hit a wall at code 7. In order to get the correct weapon you have to pray that RNGesus will show mercy, or save scum. It’s arguably the worst weapon crafting system in a video game.
Find a weapon crafting guide, and learn how it all works. There’s also a weapon discrimination tool available online, use it to get rank 1 weapons. You can start creating end-game weapons at code 6.
The biggest annoyance in doing it solo is the fact you’ll have to run the final boss a few times to raise will’o element on your weapons. Other then that, there’s the standard low drop rate for some components. However, most of them can be bought from a vendor, so its faster to farm EP instead. Overall, its not nearly as bad as some people describe.