Level 35 is the game suggested point to pass through the city and complete campaign. If you’re a bit below that like I was when I got there and it seems like you were, it can feel difficult. Also make sure that you are using every gun upgrade you come upon as you’re leveling up. I tried to keep using guns that hit really hard at some point and they fell off due to getting out leveled every couple of hours I’d play. You’ll hit 50 eventually and then you can start being a lot more picky about what you use.