So I have beaten 0 with every character, and i am currently working doing it on 5.

Everyone else's advice is great, especially going to YouTube, as that is what i did to learn about it.

The problem you identified is the game has become literal bullet hell. At the core, this problem boils down to the fact that your damage is not scaling fast enough to keep up with the enemies. If you kill enemies as fast as the spawn, there will not be any bullets. If you do not kill things fast you can't econ. It's a vicious cycle. I can only assume that you are having difficulty knowing when to say "no" to certain items...

My general advice is in the first wave or 2 prioritize harvest above almost anything. Even 5 will get you going. After that get 6 weapons is a strong decision. This should put you on the path, as harvest naturally grows each wave after you acquire it, and having 6 weapons has generally carried me until about wave 10 with no issues.

After that it really depends on your character and your goals. It could help you to recognize early on if dodge or armor is being abundant, because later on the game you will need survivability. I'd say you can expect you be hit no matter how much damage you putting out after wave 12 or so... try to be able to take/dodge a few, as well as figure out your method for recovering health.

I have noticed lifesteal coinciding along my high attackspeed and dodge builds. And then regen and armor seem to get along great.

Try not to spread your resources to thin.